Rostov-on-Don, where armoured military vehicles have been seen on the streets, is over 1,100km (680 miles) south of Moscow.

Yevgeny Prigozhin, the head of the Wagner mercenary group, says his forces now control the city.

Residents of the broader Rostov region bordering Ukraine have been told to stay indoors because security forces are operating in the area.

Wagner mercenary group boss Yevgeny Prigozhin has vowed to remove the leadership of Russia's military and says his forces have crossed into Russia from Ukraine.

Wagner troops had been fighting in Ukraine alongside the Russian army.

Russian President Vladimir Putin will make a televised address shortly, the Tass news agency quotes a Kremlin spokesman as saying.

Frank Gardner
Security Correspondent

This could go one of several ways. For Ukraine, the timing of Russia’s troubles could not be better.

Just as Kyiv was adjusting to the prospect that its long-promised counter-offensive was unlikely to expel Russian forces this year, or maybe even next, along comes a lifeline.

With the Kremlin and Wagner now preoccupied with their own struggle, this removes some of the most effective Russian fighting forces from the battlefield.

Even more importantly, this challenge to Moscow could seriously undermine morale amongst its troops manning all those fortified defensive lines that Ukraine is trying to break through.

In an ideal world this all ends with Russia cancelling the war and pulling its troops out of Ukraine. But it’s unlikely to be that simple.

Some in the West may worry about a possible collapse of the Russian state further down the line, a sobering thought for a nation with more than 4,000 nuclear warheads.

Another possible outcome is that Prigozhin’s challenge to the Kremlin will be short-lived, his forces made to surrender.

But whatever happens next, the genie is out of the bottle.

The mercenary boss who scored Russia its only significant success in months, at Bakhmut, has told the Russian people the whole pretext for the invasion was a lie, that the threat from Ukraine was invented.

Image source, Reuters

The Reuters news agency has filed some images of Wagner fighters in the southern Russian city of Rostov-on-Don, which Wagner boss Yevgeny Progozhin says his forces now control.

Image source, Reuters

Image source, Reuters

Image source, Reuters

Things are developing rapidly in southern Russia at the moment, so here's a quick roundup of what we know so far:

• Wagner mercenary group boss Yevgeny Prigozhin says his forces control key sites in the southern Russian city of Rostov-on-Don
• He has posted video in which he says Wagner forces will continue to blockade the city and move on to Moscow unless defence chiefs Sergei Shoigu and Valery Gerasimov come and meet him
• Earlier footage showed armed men in Rostov-on-Don surrounding a government building with troops and tanks
• Anti-terror measures have been introduced in Moscow, with armoured vehicles seen on the streets
• The M4 highway connecting Moscow and Rostov has been closed at the border between Lipetsk and Voronezh regions, the Lipetsk governor says

All this follows a dramatic escalation in Russian military infighting on Friday evening:

• The Russian authorities have accused Wagner boss Prigozhin of armed rebellion
• Prigozhin has accused the Russian military of carrying out a deadly missile strike on Wagner troops in Ukraine, which the defence ministry denies
• Prigozhin has vowed to "go all the way" and topple the Russian military leadership, but denied he was carrying out a coup

Video has been posted claiming to show Yevgeny Prigozhin inside Russia's southern military headquarters in Rostov-on-Don.

In one video Prigozhin says his troops will blockade the city and move on to Moscow unless Defence Minister Sergei Shoigu and General Valery Gerasimov come to meet them.

"We have arrived here, we want to receive the chief of the general staff and Shoigu. Unless they come, we'll be here, we'll blockade the city of Rostov and head for Moscow," he says.

In separate footage posted to a pro-Wagner channel, Prigozhin is seen sitting between two generals, one of whom is Army Lieutenant-General Vladimir Alekseyev who had earlier posted a video urging the Wagner boss to stand down.

Adam Levy

Yevgeny Prigozhin’s declared move against the Russian military leadership is "almost certainly an attempted coup", the former US Ambassador to Ukraine Bill Taylor has told the BBC.

“This is serious for Putin. The Russians are taking this very seriously, and it sounds like they should.”

But Taylor, who was in Ukraine last month, says the Wagner boss is unlikely to succeed.

“The ministry of defence forces have the equipment, have the aircraft, have the tanks – they have the military equipment in numbers – to put down Prigozhin's forces," he says.

But the Russian infighting could potentially help Ukraine in its fight to defeat Russia's invasion, Taylor said.

"If the Russians are now worried about their chain of command, then the Russians that are supposed to be fighting the Ukrainians may be distracted and may even run."

Image source, Reuters

The US thinktank the Institute of War (ISW) believes Yevgeny Prigozhin is aiming to force a leadership change within the Kremlin's defence ministry - but suggests he is "unlikely to succeed" because the Kremlin appears opposed to his action.

However any Wagner attempt to take control of Rostov-on-Don could have an impact on the war in Ukraine because Rostov is home to the Russian southern military district command, which has been key to Russia's war efforts.

The southern military district’s 58th combined arms army is “currently decisively engaged in defensive operations against Ukrainian counteroffensives in southern Ukraine, and the command centre for the Russian joint group of forces in Ukraine as a whole”, it says.

The M4 highway has been closed at the border between Lipetsk and Voronezh regions, the Lipetsk governor says.

Earlier he said a Russian military convoy was on the highway.

The M4 connects Moscow to southern regions including Rostov and its main city Rostov-on-Don, where armed men have been seen.

It follows Wagner mercenary group boss Prigozhin's vow to topple Russia's military leadership

Anti-Kremlin figure Mikhail Khodorkovsky has urged Russians to support Wagner chief Yevgeny Prigozhin who, as we've been reporting, has vowed to bring down Moscow's military leadership.

Khodorkovsky, who was once Russia’s richest oligarch, said: "We need to help now, and then, if necessary, we will fight this one, too."

After falling out with Vladimir Putin, he spent 10 years in a Russian prison and recently called for tougher sanctions against the Russian president.

Khodorkovsky said it was important to back "even the devil" if he decided to take on the Kremlin.

"And yes, this is just the beginning," he said.

Image source, EPA

Moscow's mayor has announced anti-terrorist measures are being taken to reinforce security in the capital city.

Writing on Telegram Sergey Sobyanin wrote: "In connection with the incoming information, anti-terrorist measures are being taken in Moscow aimed at strengthening security measures.

"Additional control on the roads has been introduced.

"It is possible to limit the holding of public events. Please consider the measures taken with understanding."

Things appear to be moving very fast in the city of Rostov-on-Don, where footage on local Telegram channels appear to show armed forces surrounding a government building.

The footage - which has not been verified by the BBC - shows armed men in uniform walking around the building's perimeter, as armoured vehicles surround it, including two tanks with their guns trained on it.

Reuters news agency said it had verified the location as the police headquarters building.

Meanwhile the local governor Vasily Golubev urged residents to avoid the centre of Rostov-on-Don.

"In connection with the current situation, I ask you to refrain from traveling to the city center and, if possible, not to leave your homes," he wrote on Telegram

Bernd Debusmann Jr
BBC News, Washington DC

The upsurge of Russian infighting is being watched closely around the world, including by former US Deputy Assistant Secretary of Defense and CIA officer Mick Mulroy.

He told the BBC Wagner boss Prigozhin was now presenting a serious challenge to President Putin.

"It says a lot that Russia had to rely on mercenaries to try and salvage its strategically disastrous invasion of Ukraine. It says even more that Prigozhin is now acknowledging that it was, in fact, unprovoked and that the Russian people were lied to from the beginning," he says.

If Wagner forces can actually threaten Putin's control of the country, Russia may have to "reorient its military strength toward self-preservation and away from its efforts to stop the Ukrainian counter-offensive", he tells me.

"Even if this attempted coup fails, it emphatically makes the point that those closest to this war know it was a terrible mistake."

The government of Voronezh region has urged people to avoid the M4 motorway connecting Moscow to southern regions of Russia because a military convoy is moving along it.

Writing on Telegram, officials also claimed the situation is under control and plans are in place to protect public safety.

The M4 motorway runs for over 1,500 kilometres (930 miles), linking the Russian capital to the cites of Voronezh, Rostov to the east of Ukraine and Krasnodar.

Videos allegedly showing Wagner fighters moving in a convoy of military vehicles along the road towards Rostov have been posted on social media, but the BBC is not able to verify them.

Joe Inwood
BBC News Correspondent, formerly in Ukraine

Image source, Reuters

Throughout this war, there has been another battle raging, one between two men at the heart of Russian power.

Yevgeny Prigozhin, leader of the notorious Wagner mercenary group, and Sergei Shoigu, the Russian Defence Minister, have been jostling for position.

It initially centred around Bakhmut, an unremarkable town in the middle of Ukraine’s eastern Donbas region. While the regular Russian army struggled elsewhere, the Wagner group managed to make gains.

Prigozhin used his forces’ success as an opportunity to build his own profile – and criticise the troops under Shoigu.

He accused the Ministry of Defence of denying his men ammunition, even threatening to pull out. When victories occurred, both sides tried to take credit.

Prigozhin always took care to avoid criticising President Putin himself, but was scathing of those around him.

People I spoke to at the time suggested he was sailing very close to the wind, potentially overplaying his hand.

If the emerging reports are to be believed, it seems he has decided to go all in.

The governor of Russia's Lipetsk region is asking residents not to travel south, as security in Russia tightens.

Lipetsk is around 280km (175 miles) to the north-east of the nearest Ukrainian border and more than 500km north of Rostov.

Writing on Telegram, Igor Artamonov says security measures in the region are being tightened, with a particular focus on protecting critical infrastructure facilities.

He also asks people to remain calm and trust information from official sources.

We now have some footage of increased security measures in Moscow which has just come to us from Reuters.

It comes after the authorities there accused the Wagner mercenary group boss Yevgeny Prigozhin of inciting armed mutiny.

You can watch it below by clicking Play.

Thomas Mackintosh
Live reporter in London

Lets take a step back and recap the developments of the past few hours.

There has been a dramatic escalation of infighting between Russia's Wagner mercenary group and the Russian military.

• Wagner boss Yevgeny Prigozhin called for rebellion against the army, accusing it of carrying out a missile strike against his forces, something the defence ministry denies
• The Kremlin says Prigozhin is now being investigated for inciting mutiny
• Security has been tightened in Moscow and in the city of Rostov-on-Don, about 100km to the east of the Ukrainian border
• Prigozhin says his forces have entered Russia from Ukraine - although we have not yet seen evidence of this. He has also claimed his forces have shot down a Russian helicopter, again without providing evidence
• The governor of Russia's Rostov region has told residents to stay indoors and remain calm
• Russian president Vladimir Putin is being updated on the situation, as is US President Joe Biden

The Wagner boss has just posted a fresh entry on the Telegram social media platform claiming that his forces have shot down a Russian helicopter.

He claims the helicopter "opened fire on a civilian convoy" - although he did not give details as to where this incident is alleged to have happened.

Earlier he said his forces had entered Russia's Rostov region, to the east of Ukraine.

In the last few moments the governor of Russia's Rostov region - which borders Ukraine to the east - has urged citizens to stay indoors.

Writing on Telegram Vasily Golubev said: "The current situation requires the maximum concentration of all forces to maintain order.

"Law enforcement agencies are doing everything necessary to ensure the safety of residents of the area.

"I ask everyone to stay calm and do not leave the house without the need."

Wagner group boss Yevgeny Prigozhin earlier claimed without providing evidence that his forces had crossed from Ukraine, where they had been fighting, into Russia.

Russian military vehicles have been deployed in the regional city of Rostov-on-Don, about 100km to the east of the border with Ukraine.