The UK's terror threat level has been raised from severe to critical, the highest level, the prime minister has announced, after a suicide bombing in Manchester which killed 22 people.
A suicide bomber killed 22 people, including children, as an explosion tore through fans leaving an Ariana Grande pop concert in Manchester on Monday night.
Some 59 people were also injured in the blast when the attacker, who authorities identified as Salman Abedi on Tuesday afternoon, detonated an improvised explosive device at the Manchester Arena.
A 23-year-old man was arrested in south Manchester on Tuesday afternoon in connection with the attack.
On Tuesday night, Theresa May confirmed that the UK's terror threat level had been raised to critical - meaning that another attack may be imminent.
Military personnel will take over from armed police at various sites around the country as part of increased security.
Three of the victims have been named as college student Georgina Callander, eight-year-old schoolgirl Saffie Roussos and 26-year-old John Atkinson.
Many families are still waiting to hear news of loved ones who attended the concert.
Greater Manchester police chief Ian Hopkins said on Tuesday morning that investigators believed the attack was conducted by a single terrorist.
He said it was the 'most horrific incident' the area has had to face.
Emergency services have confirmed that casualties were taken to eight different hospitals, with 12 children aged under 16 taken to Manchester Children's Hospital.
Prime Minister Threresa May visited Manchester on Tuesday, visiting the children's hospital and meeting emergency services.
She condemned the 'appalling, sickening cowardice' of the attack and praised the bravery of those who helped.
MORE: Prime Minister to host emergency Cobra meeting after Manchester terrorist attack
Police were initially called to reports of an explosion at the Manchester Arena at 10.33pm, shortly after US singer Ariana Grande had finished her performance.
Victims described being thrown by the blast that scattered nuts and bolts across the floor.
Witnesses said they heard two loud bangs inside the arena coming from the area of the bars.
One fan at the concert with his sister described how the apparent explosion happened as fans were leaving the arena.
Majid Khan, 22, said: 'A huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena.
'It was one bang and essentially everyone from the other side of the arena where the bang was heard from suddenly came running towards us as they were trying to exit.'
Oliver Jones, 17, who attended with his 19-year-old sister, said: 'The bang echoed around the foyer of the arena and people started to run.
MORE: Suicide bomber died in Manchester terrorist attack at Ariana Grande concert
'I seen people running and screaming towards one direction and then many were turning around to run back the other way.'
The injured are being treated at hospitals across Greater Manchester, and a hotline has been set up for those with concerns over loved ones who remain unaccounted for.
Ariana Grande has reportedly cancelled her upcoming shows in London and she tweeted: 'broken. from the bottom of my heart, i am so so sorry. i don't have words.'
Local reaction
Norfolk County Council, Suffolk County Council, Norwich City Council, West Norfolk Council, Broadland District Council, Attleborough Town Council and Great Yarmouth Borough Council are among local authorities which have lowered their flags in tribute to the victims.
A tweet from County Hall stated: 'We're flying the Union Flag at half mast today in honour of the victims of the Manchester terror attack #StandTogether'.
At Broadland, leader Andrew Proctor called for a minute's silence ahead of a meeting of the council's cabinet, while in Norwich, the mayor-making ceremony on Tuesday afternoon was preceded with a minute's silence.
The Norfolk Constabulary and Norfolk Fire and Rescue Service flags were also flown at half-mast in respect to those affected by the attack.
MORE: Authorities identify Manchester Arena suspected bomber
Bishop of Norwich, the Rt Revd Graham James, described the attack as 'an act of hatred' and said his thoughts are with the people of Manchester.
'Everyone will be horrified by what has happened in Manchester,' he said. 'The targeting of young people adds to our sense of outrage. Who knows the intention of the perpetrator but it must be to undermine the social cohesion of our society.
'It was an act of hatred. It can only be countered by love - and it is clear that people in Manchester have responded by countless acts of generosity and compassion.
'While we pray for the victims, the injured, the traumatised and their families, I hope we will also resolve to be as generous and compassionate as so many people in Manchester have been in these past hours. Our hearts go out to them all.'
A summary of events so far
• 22 people, including many children, have been killed and 59 injured in the blast.
• The attack was carried out by a lone male suicide bomber who detonated an improvised explosive device. He died at the arena.
• Security services believe they know his identity and are investigating whether he acted alone or was part of a network.
• A 23-year-old has been arrested in South Manchester in connection with the incident.
• Islamic State has claimed responsibility for the attack.
• The first victim has been named as college student Georgina Callander. Eight-year-old schoolgirl Saffie Roussos was also killed.
• Speaking after chairing a Cobra meeting on Tuesday morning, Prime Minister Theresa May said the 'callous' attack was 'among the worst terrorism we have experienced in the United Kingdom'.
• A vigil will be held at 6pm on Tuesday in Albert Square, Manchester.
• The explosion took place at Manchester Arena as people began streaming from the doors after a show by American singer Ariana Grande.
• Greater Manchester Police said they were called to the venue at around 10.33pm and approach roads were closed.
• Manchester Victoria station was evacuated and trains cancelled until at least the end of Tuesday.
• The injured - including 12 under the age of 16 - are being treated at eight hospitals across Greater Manchester, Chief Constable Ian Hopkins said.
• President Donald Trump expressed his 'deepest condolences' to the victims and branded the attackers 'evil losers'.
• Extra police officers have been put on duty in London in the wake of the attack, Metropolitan Police Commissioner Cressida Dick announced.
• Andy Burnham, the Greater Manchester metro mayor, called the atrocity 'an evil act'.
• All national General Election campaigning was suspended after the explosion.
• A controlled explosion was carried out on Tuesday afternoon, as part of the investigation, at an address in Fallowfield.
• Theresa May has visited Manchester Children's Hospital, Downing Street has said.
• Authorities has identified the suspected bomber as Salman Abedi, the Press Association reports.
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