Prime Minister Theresa May warned that intelligence services believe an attack may be "imminent."
Hours earlier, police identified 22-year-old Salman Abedi as the suspected suicide attacker who detonated a bomb as throngs of teenagers poured out of an Ariana Grande concert
in Manchester, killing 22 people.
- Police have carried out two raids in Manchester.
- Queen Elizabeth II described bombing as "act of barbarity."
- 59 people injured, some in life-threatening situations.
- US President Donald Trump slams attackers as "losers."
- Ariana Grande suspends her world tour.
The suspect apparently acted as a lone attacker and died in the blast Monday night, police said, which left the wounded and the dead scattered across the arena's bloodied entrance and sent screaming girls running for cover.
The bombing has drawn condemnation and horror from around the world as a heinous assault targeting children.
Teen concertgoer Olivia Campbell, whose mother spoke to CNN during an agonizing wait
for news from her daughter, is the latest victim to have been confirmed dead.
Fifteen-year-old Olivia had gone to the concert with her friend Adam to celebrate his birthday. She is one of four victims, including eight-year-old Saffie Rose Roussos
, who have been identified.
While police believe Abedi
acted alone, May warned that investigators could not discount the possible involvement of a "wider group" of terrorists.
The raising of the terror threat level means "not only that another attack remains highly likely but that a further attack may be imminent," she said.
It will enable the government to deploy armed military personnel of support of police and other security forces, particularly in the guarding of key sites, concerts and sporting events, May said.
"I do not want the public to feel unduly alarmed," she said, calling the response "proportionate and sensible" to the threat level.
The blast marked the deadliest terror attack on British soil since the 2005 London bombings.
Claim of responsibility
ISIS claimed responsibility for the attack but offered no evidence. The terror group said on its Telegram channel Tuesday that a "soldier of the caliphate" was able to "plant explosive devices" at the arena, a US counterterrorism source told CNN.
ISIS routinely claims attacks it has no proven connection to.
Authorities have discovered no evidence of a link between the attacker and an established terror group, a British counterterrorism official told CNN.
A US counterterrorism official said bombing "looks much like" an ISIS attack but that American intelligence officials were working with British counterparts to determine more.
No determination has been made on the sophistication of the explosive device or what chemicals were involved, the official said.
Remembering the victims
"We will stand together to say that this city is greater than the force that aligns itself against it," David Walker, Bishop of Manchester, told the crowd.
"We are sending a signal not just to Manchester, but across the world that you can not defeat us because love in the end is always stronger than hate."
Eight-year-old Saffie Rose Roussos from Leyland has been named as one of the fatalities, the Lancashire County Council confirmed.
Chris Upton, the head teacher at the Tarleton Community Primary School, described her as "simply a beautiful little girl" who was "quiet and unassuming with a creative flair."
Georgina Callander, 18, was also killed, according to her school, the Bishop Rawstorne Church of England Academy.
"All of our students will gather together today for a time of prayer and reflection and to give thanks for the life of Georgina," the school said.
At least 12 victims aged 16 or under were being treated at a children's hospital for serious injuries, some of them fighting for their lives, a Manchester health official said.
Grande, who had just finished the first of three scheduled UK performances, tweeted about her devastation several hours later: "broken. from the bottom of my heart, i am so so sorry. i don't have words."
The pop star has suspended her "Dangerous Woman" tour
following the attack, a source with knowledge of the situation told CNN. Grande was scheduled to perform in London and across Europe through mid-June.
Queen Elizabeth II said Tuesday that "the whole nation has been shocked."
Around 400 police were deployed overnight following the attack, and on Tuesday, large groups of armed police were seen in several parts of the city. Security was boosted in London.
Trump calls attackers 'losers'
US President Donald Trump slammed the attack, saying that terrorists were "losers."
"So many young beautiful innocent people living and enjoying their lives murdered by evil losers in life. I won't call them monsters because they would like that term. They would think that's a great name. I will call them from now on losers because that's what they are," he said. "This wicked ideology must be obliterated."
The US Department of Homeland Security said it was "closely monitoring" the situation.