Theresa May will chair a meeting of the Government's emergency Cobra committee on Tuesday following the Manchester explosion which is being treated as an "appalling terrorist attack".
The Prime Minister said the authorities were working to establish what happened at the Manchester Arena, after 22 people were confirmed dead following reports of an explosion at an Ariana Grande concert.
Mrs May said: "We are working to establish the full details of what is being treated by the police as an appalling terrorist attack.
"All our thoughts are with the victims and the families of those who have been affected."
A Tory source said the Prime Minister's General Election campaign was being suspended.
The Cobra meeting is expected to take place at around 9am, Downing Street said.
Flags in Downing Street have been lowered to half-mast in honour of those killed and injured.
A suicide bomber killed 22 people, including children, as an explosion tore through fans leaving a pop concert in Manchester.
Some 59 people were also injured when the blast caused by an improvised explosive device carried by the attacker detonated at the Manchester Arena.
Announcing that the death toll had risen, Greater Manchester Police chief constable Ian Hopkins said: "What I can confirm is that there are children among the deceased."
He said: "This has been the most horrific incident we have had to face in Greater Manchester and one that we all hoped we would never see.
"Families and many young people were out to enjoy a concert at the Manchester Arena and have lost their lives.
"Our thoughts are with those 22 victims that we now know have died, the 59 people who have been injured and their loved ones.
"We continue to do all we can to support them."
Mr Hopkins said a "fast-moving investigation" had established the attack was conducted by one man, although detectives are working to establish if he was "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."
Police were 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.
More than 240 calls were made to the emergency services, with responders including 60 ambulances flooding the area.
More than 400 police officers were deployed as part of the operation, with a visible presence remaining on the streets of Manchester on Tuesday.
The dozens of victims injured in the attack 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.
A large cordon remains in place around the arena and nearby Manchester Victoria Station, which was evacuated during the incident and remains closed, while forensic investigators gather evidence.
Police have appealed for concert-goers and witnesses to provide police with footage from the scene if they believe it can assist the probe.
Meanwhile anyone who witnesses suspicious activity is urged to report it to the anti-terrorist hotline.
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