Britain has increased its security threat level to 'critical' meaning a further attack may be 'imminent'
Britain has increased its security threat level to "critical" from "severe" following the suicide attack in Manchester that killed 22 people, including children, Prime Minister Theresa May said on Tuesday.
"It is a possibility we cannot ignore that there is a wider group of individuals linked to this attack," she said.
May also said members of the armed forces would boost security at key sites and military personnel might be deployed at public events such as concerts and sports events.
May said the independent body which sets the threat level had recommended it be raised after a man named by police as Salman Abedi - a 22-year-old British man of Libyan descent - set off an improvised bomb on Monday night as crowds streamed out of a concert.
"It is now concluded on the basis of today's investigations that the threat level should be increased for the time being from severe to critical," she said in a televised statement following a meeting of the government's crisis response committee.
"This means that their assessment is not only that an attack remains highly likely but that a further attack may be imminent."
Abedi was known to the security services but was not flagged as high risk, police said.
The Guardian quoted shocked members of Manchester's Libyan community describing him as having been a "quiet" and "respectful" man.
However, Mohammed Saeed, the imam of the mosque where Abedi prayed in Didsbury, said that the young man had looked at him "with hate" after he gave a sermon criticising the Islamic State (IS) group and other militant groups in Libya in 2015, the paper reported.
May said the bomber, who struck at about 10.30pm at the Manchester Arena on Monday, aimed to cause "maximum carnage" by detonating his bomb outside one of the exits.
IS claimed the attack through its propaganda wing, Amaq, saying it had scored a "great victory" against the "crusaders" of Britain.
One of the youngest victims was named on Tuesday as Saffie Rose Roussos from Leyland, near Preston. She was eight years old.
Police on Tuesday said they conducted raids in Manchester connected to the attack, and conducted a "controlled explosion" in the Fallowfield area where Abedi lived. A 23-year-old man was arrested in the south of the city.
People attend a vigil in Manchester on 23 May 2017 in solidarity with victims of attack at the Manchester Arena (AFP)
"It is now beyond doubt that the people of Manchester and of this country have fallen victim to a callous terrorist attack, an attack that targeted some of the youngest people in our society with cold calculation," May said.
On Tuesday evening thousands of people attended a vigil in central Manchester in sombre but defiant mood.
"There's hard times again in these streets of our city, but we won't take defeat and we don't want your pity, because this is the place where we stand strong together with a smile on our face, Mancunians forever," local poet Tony Walsh said in a poem he read to the crowd that drew loud cheers and applause.
British police do not routinely carry firearms, but London police said extra armed officers would be deployed at this weekend's soccer cup final at Wembley and rugby at Twickenham. Security would be reviewed also for smaller events.