Russian forces launch a full-scale assault on Ukraine, with its military attacking the country from the north, east and south
Ukrainian President Zelensky says 137 Ukrainian citizens - both soldiers and civilians - died on Thursday
People in the capital, Kyiv, and elsewhere are trying to flee - some 100,000 have left so far, the UN says
There are also renewed reports of explosions in the port city of Mariupol, home to half a million people
Russians seize control of the Chernobyl complex - site of the world's worst nuclear disaster
UK and US announce fresh new sanctions on Russia, including asset freezes on banks
President Vladimir Putin defends his move, saying there was no other way to defend Russia
But US President Joe Biden says Putin's aggression will cost Russia dearly
The London Stock Exchange's leading FTSE 100 index plunges more than 200 points, or 2.7%, within moments of opening in reaction to Russia's invasion of Ukraine.
Here's more from European Commission Chief Ursula von der Leyen.
She says that later today, European leaders will be presented with "a package of massive and targeted sanctions" for approval.
"With this package, we will target strategic sectors of the Russian economy by blocking their access to key technologies and markets. We will weaken Russia's economic base and its capacity to modernise," she says.
"In addition, we will freeze Russian assets in the EU and stop the access of Russian banks to the European financial market. Like with the first package of sanctions, we are closely aligned with partners and allies. These sanctions are designed to take a heavy toll on the Kremlin's interests and on their ability to finance the war."
Image source, Getty ImagesSmoke rises from a military airport in Chuhuyev near Kharkiv, in eastern Ukraine
Junior UK Foreign Minister James Cleverly has been telling the BBC and other British media that the UK is bringing forward an unprecedented and coordinated sanctions package.
Sanctions would start today and more will be announced in waves in the coming days.
They would be the largest and most severe sanctions Russia has ever seen, he told Sky News.
Cleverly told BBC Breakfast that Ukrainians "should know that the UK stands with them".
He said the UK would be providing "direct assistance" to Ukraine in the defence of their country.
"This has long been planned by Vladimir Putin. This is an invasion and it is completely unprovoked and unjustified."
European Commission President Ursula von der Leyen, speaking in Brussels, condemns Vladimir Putin for "bringing war back to Europe".
She announces a package of "massive" sanctions will be proposed later in a bid to weaken Russia's economic base and its "capacity to modernise".
Paul Adams
BBC diplomatic correspondent, Kyiv
Image source, ReutersUkrainian police inspect the remains of a missile that landed in the street in Kyiv
Ukraine says that Russian military vehicles have crossed into the country in multiple locations, including from Belarus in the north and Crimea in the south.
Other crossings have taken place in the east, in the region of Kharkhiv and Luhansk. According to Ukraine’s border service, Russia opened fire with artillery before sending military vehicles across.
In an assessment by western intelligence officials 10 days ago, it was estimated that for Russia to launch a successful attack on Kyiv (from the north), Ukraine’s most capable military units (in the east, facing the Donbas) would need to be encircled and/or neutralised.
The evidence so far suggests that the attack is coming from every direction.
Seven people are known to have died in bombing by Russian forces, Ukrainian police say.
Officials say an attack on a military unit in Podilsk, outside Odessa killed six people and wounded seven.
Nineteen people are also missing.
One person died in the city of Mariupol, they added.
Ukraine conflict: Sirens heard during BBC reporter's broadcast in Kyiv
Our correspondent Nick Beake was reporting on the sound of explosions in the Ukrainian capital Kyiv as an air raid siren started wailing out across the city.
Explosions have been heard on the outskirts of Kyiv while gunfire has been reported near its main airport.
We reported earlier a statement by the Ukrainian military that its air force was fighting off an air attack by Russia.
But Russia's defence ministry is quoted by Russia's Interfax news agency as saying that the "Ukrainian armed forces' air defence has been suppressed".
The ministry adds that "Ukrainian border forces put up no resistance to Russian units".
There has been no independent confirmation of these claims.
Boris Johnson is about to chair an emergency government Cobra meeting, Downing Street says, before making a statement to the House of Commons later today.
The committee will meet at 07:30 GMT to discuss the UK's response to the "horrific attacks". The PM is also expected to join a call with G7 leaders.
Johnson has already said Vladimir Putin has "chosen a path of bloodshed and destruction" with his attack on Ukraine and that the UK and its allies would respond "decisively".
Nations in Europe have been quick to condemn Russia's action.
A German government spokesman said Chancellor Olaf Scholz had spoken to Ukrainian President Volodymyr Zelenskiy and assured him of solidarity.
"We have a land war in Europe that we thought was only to find in history books. It is a flagrant breach of international law," economy minister Robert Habeck added in a statement.
UK Prime Minister Boris Johnson said he was "appalled" by the "unprovoked attack" and Britain would "respond decisively".
Italian Prime Minister Mario Draghi said the action was "unjustified and unjustifiable".
"We are working with European and Nato allies to respond immediately, with unity and determination."
Meanwhile, Polish government spokesman Piotr Muller said it was "time to reinforce Nato's eastern flank", adding: "We expect such decisions to be taken".
Russian military convoys have crossed into Ukraine in northern Chernihiv and Sumy regions, and in the eastern Luhansk and Kharkiv regions, Ukraine's border guard service (DPSU) has said.
It says the Russian offensive was preceded by artillery fire.
There were injuries among Ukrainian border guards, the DPSU says.
It says Ukraine's border guards and the armed forces are "taking all measures to stop the enemy".
Russia has begun what President Vladimir Putin has called a "special military operation" in Donbas, ending weeks of speculation about Russia's intentions in Ukraine.
Here's what we know so far:
•In a televised address to the Russian people, Putin said that the operation is aimed at the "demilitarisation and denazification" of Ukraine.
•Missile strikes and explosions have been reported in several parts of Ukraine, including Kyiv and Kharkiv.
•Ukraine's Foreign Minister accused Russia of starting a full-scale war and implored the UN to "do everything possible" to stop it.
•US President Joe Biden has condemned the attack and vowed to hold Russia accountable.
•Leaders from the UK, European Union, UN and NATO have also strongly condemned the Russian offensive.
Image source, Reuters
Nick Beake
BBC News, Kyiv
On the streets of Kyiv this morning it is quiet and more military personnel are noticeable than would usually be seen here.
One tourist told us his hostel had thrown him out after air raid sirens and warnings were sounded by the government here.
He's not the only one with suitcases on the streets - some people told us that they didn't know what to do. Many didn't want to stop and talk as they hurried onwards.
Others told us they had lost connectivity on their mobile phones.
There are reports of queues in supermarkets and we have seen people withdrawing money at ATMs "just in case".
Image source, GetThe Ukrainian armed forces say they have shot down five Russian planes and a helicopter.
"Keep calm and believe in Ukraine defenders", the statement from Ukraine's forces says.
Russia's defence ministry has denied that its aircraft were shot down.
Just over two hours ago, the Russian President Vladimir Putin announced the military action.
Many people in Kyiv have rushed to underground metro stations to take shelter. Others have boarded buses to leave the city, and there are queues of cars trying to get out.
Image source, Getty ImagesCrowds at a metro station in Kyiv as air raid sirens ring out across the city. Many people are carrying bags and suitcases
Image source, Getty ImagesA father reassures his son as his family takes refuge in a metro station
Image source, Getty ImagesGirls on their phones, taking shelter inside the underground station
Image source, Getty ImagesPeople seen embracing on the streets
Image source, Getty ImagesFamilies head into the metro station on Thursday morning
The BBC's eastern Europe correspondent Sarah Rainsford, reporting from the city of Kramatorsk in Ukraine's Donbas region, says residents are "shocked and horrified to imagine what might be coming".
She says the situation in the area is "completely unclear".
"People were out on the streets last night in this city - they were waving the Ukrainian flag. They said this was their land. They were going nowhere....
"This is what people have been expecting, they have been waiting for, but no-one here can quite believe it's actually happening."
Image source, ReutersCars queue to exit Kyiv in the early morning of Thursday
Vladimir Putin announced the military operation into Ukraine at 5.55am Moscow time - and minutes later the first shelling and missiles were launched into Ukraine, according to reports.
In the capital of Kyiv, an emergency siren has gone off, and pictures show streams of cars clogging up an expressway as people flee the city.
Social media testimonies speak to a growing sense of panic, with some saying they are being rushed into bomb shelters and into basements. Television footage has showed people praying in the streets, huddled in groups.
Guardian journalist Luke Harding in Kyiv says on Twitter, external that there are few people on the streets, and people are queuing up at cash machines.
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Dmytro Kuleba, Ukraine's foreign minister, has tweeted a "to do list" for the international community in response to Russia's attack.
He says "the world must act immediately" and the "future of Europe & the world is at stake".
Kuleba calls for immediate "devastating sanctions", including banning Russia from using the SWIFT financial transaction system.
He also says the world should "fully isolate Russia", while providing Ukraine with weapons and equipment, financial assistance and humanitarian help.
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The Ukrainian armed forces has posted a statement, external saying that Russian military began "intensive shelling" of its units in the east of the country.
Russia also launched missile strikes on Boryspil airport near Kyiv and several other airports, the statement said.
It said the Ukrainian air force is fighting off an air attack by Russia.
The statement denied reports about Russian paratroops in the southern port city of Odesa.
Multiple reports are now quoting Ukrainian officials as saying troops in neighbouring Belarus are joining the Russian attack, meaning the offensive is now also coming from Ukraine's north.
Belarus has long been allied with Russia. Analysts describe the small country as Russia's "client state".
The attack from the north adds to Russia's attacks on Ukraine's east, and Russian troop movement on Odessa in the south.