Security services are working to find out whether a terrorist attacker in Manchester acted alone or was part of a wider group, Prime Minister Theresa May says.
May, speaking from outside Downing Street, said: "It is now beyond doubt that Manchester has fallen victim to a callous terrorist attack, an attack that targeted some of the youngest people in our society with cold calculation."
She said it was “one of the worst terror incidents we have ever experienced in the United Kingdom.”
"This attack stands out for its appalling, sickening cowardice," she added.
A suicide bomber killed 22 people, including children, as an explosion tore through fans leaving an Ariana Grande concert at Manchester Arena on Monday night. Some 59 people were also injured in the blast.
May says a terrorist detonated an improvised explosive device at a time and place to cause "maximum carnage."
She says police believe they know the identity of the attacker, but that his name would not be released yet.
May says security services "believe the attack was carried out by one man" but need to find out whether "he was acting alone or part of a wider group."
She says if others are responsible for the attack, they will be brought to justice.
May says those injured are in eight hospitals across Manchester, some with "life threatening conditions."
May will travel to Manchester later on Tuesday to meet the chief constable, the mayor and the emergency services.
The election campaign has been suspended, she says.
She will chair another COBRA meeting on Tuesday. May says the terror threat level remains at 'severe' but will continue to be assessed.
"There will be difficult days ahead. Let us remember those who died, and think of those who helped.
"Our way of life will always prevail."
May, who is now in Manchester, told journalists that the police will be looking at venues to ensure people and their children going to concerts and festivals over the summer are safe.
“What is clear is that we are ensuring that resources are available for the police in order to be able to do the job that they need to do.
“This was a horrendous attack, absolutely horrific, barbaric in nature, but what I’m also clear about is that we will not let the terrorists win. Our values will prevail.”