The chief constable says they are treating it as a terrorist incident and they believe the attack was carried out by one person.

He says the attacker died at the scene.

He believes the attacker had a device which he detonated at the scene.

He says they are trying to find out if he was acting alone.

Ian Hopkins, chief constable of Greater Manchester Police, says it is the most horrific incident they have faced in Greater Manchester.

He says more than 400 armed officers were deployed last night to the incident.

Greater Manchester Police are giving a press conference to update the situation.

They confirm the death toll has risen from 19 to 22.

Police are due to give an update on the situation shortly.

Relatives of concert-goers appeal on social media to find them.

Tom Mullen
Reporter, BBC News Online

People have described the "sheer panic" of the Manchester Arena blast that left 19 dead, more than 50 injured and survivors fleeing for their lives.

Shortly after 22:35 BST, the streets of the city centre were suddenly lit up in blue flashing lights, and helicopters buzzed overhead.

It was obvious from the beginning that this was incredibly serious.

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The Press Association's health correspondent tweets...

Nineteen people have been killed and more than 50 injured in an explosion at the end of a concert by US singer Ariana Grande at Manchester Arena.

Image source, Getty Images

Here is what we know so far.

• The arena said the explosion happened outside the venue, as people began streaming from the doors
• Greater Manchester Police were called to the scene at 22:35 BST on Monday and surrounding roads were closed
• The cause is unknown but Prime Minister Theresa May said police were treating it as "an appalling terrorist attack"
• GMP chief constable Ian Hopkins said the force would treat it as terror "until we have further information"
• If confirmed to be a terrorist attack it would be the worst in the UK since 56 people were killed in the London bombings of July 2005
• Unconfirmed reports from two unnamed US officials suggested the attack was carried out by a suicide bomber
• Eyewitnesses reported nuts and bolts strewn around the floor and the smell of explosives
• All national campaigning in the general election has been suspended
• Mrs May will chair a meeting of the government's emergency Cobra committee at 09:00
• The nearby Victoria railway station was evacuated and all Tuesday trains have been cancelled

Here is Labour leader Jeremy Corbyn's full statement:

Increased police patrols in Manchester - including at Old Trafford football stadium.

The BBC's security correspondent tweets...

Audience members scream as they realise something terrible has happened.

Amateur footage captures the sound of the blast and a flash of light outside Manchester Arena.

Forensic offiicers are continuing to examine the scene of the blast at Manchester Arena, which remains cordoned off this morning.

Image source, Getty Images

Image source, Getty Images

And here's Home Secretary Amber Rudd's full statement, via Huffington Post's executive editor of politics.

An eyewitness tells the BBC he was blown through doors by an explosion at Manchester Arena.

Footage shows a string of emergency services vehicles heading to the Manchester Arena after reports of the explosion.