Chief constable Ian Hopkins has confirmed that the death toll has risen from 19 to 22, with a further 59 injured, in what he described as "the most horrific incident we have had to face in Greater Manchester and one that we all hoped we would never see."
Mr Hopkins also explained that while they believe the attack was conducted by one man, they are still investigating.
"We have been treating this as a terrorist incident and we believe that while the attack last night was conducted by one man, the priority is to establish whether he was acting alone or as part of a network.
"The attacker, I can confirm, died at the arena.
We believe the attacker was carrying an improvised explosive device, which he detonated, causing this atrocity.
We would ask people not to speculate on his details or to share names. There is a complex and wide-ranging investigation under way."
The police are now working with the national counter-terrorist policing network and UK intelligence to try and determine more details about who carried out the attack.
Manchester's Mayor Andy Burnham has described the attack as "an evil act", but taken time to thank the emergency personnel.
"I want to thank the hundreds of police, fire and ambulance staff who worked through the night.
"I want to thank the people of Manchester: even in the minutes after the attack they opened their doors to strangers and drove them away from the attack.
"It will be that spirit of Manchester that will prevail and hold us together."
Mr Burnham said he was a minister in the home office during the London bombings in 2005.
"I remember how London felt on that day, I remember it vividly … Manchester will pull together … that’s what we are, that’s what we do. They won’t win.
"This city has dealt with difficult things in the past and we will do so now."
Ariana has reportedly been taken to hospital and TMZ says that the singer will not perform in London on Thursday, as well as putting the rest of her European tour on hold.
The singer recently tweeted her heartache over the attacks.
Two U.S officials told Reuters, on condition of anonymity, that the initial signs point to a suicide bomber - but nothing has been confirmed by authorities yet.
In scenes reminiscent of the Paris' 2015 terror attacks, which spawned #pourteouverte, people in Manchester are offering up their homes and hotels for those caught up in the aftermath with the hash tag #RoomForManchester. Cabbies are also understood to be turning off their meters and taking people home.
Officers carrying out a precautionary controlled explosion in Cathedral Garden confirm that it was abandoned clothing, not a suspicious item.
The explosion from the concert can be heard in this dash-cam video taken outside the venue.
A controlled explosion is being set off in Manchester, suggesting police could be detonating a bomb.
Families have been desperately taking to Twitter to try to find loved ones.
19 people have been confirmed dead so far and Greater Manchester Police have just issued a statement saying it's being treated as a terrorist incident until police know otherwise.
There's now footage on Twitter of a man being pulled out of a car and arrested by police as reports are swirling the Ariana Grande concert explosion might not have been accidental.
More as story develops.
An Ariana Grande concert has ended in tragedy and terror after an unidentified explosion has killed several people in Manchester in the UK.
Greater Manchester Police have said there have been a "number of confirmed fatalities" with some reports saying there were two blasts.
Eyewitnesses at the scene say there are people with blood on them running from the Manchester arena.
Watch: Emergency services arrive at the initial explosion
"Honestly worst night of my existence. Just kept running from Manchester Arena for my life," one Twitter user wrote.
It’s not yet clear how many fatalities there have been or what caused the explosion but some reports claim the blast was a blown speaker.
More as we get it.
It was pandemonium as fans began to flee the arena, which has a capacity of 21,000.
The loud bang occurred shortly after the music concert finished and the lights had turned on at 10.30pm BST.
One witness told Manchester Evening News: She [Ariana Grande] did her last song, we were in the lower tier and there was an explosion behind us at the back of the arena.
“We saw young girls with blood on them, everyone was screaming and people were running. There was lots of smoke.”
The Holiday Inn next to the Manchester Arena has been taking in "dozens of kids" who've become separated from their parents - everyone's worst nightmare.