Britain has raised its terrorism alert to critical -- the highest level -- and soldiers are being deployed to assist armed police, Prime Minister Theresa May announced Tuesday.
May addressed the country one day after a suicide bomber blew himself up at an Ariana Grande concert in Manchester, killing 22 people and wounding nearly 60. She said members of the military will be deployed along with police at what she called "big events," including football (soccer) matches and concerts.
WATCH: VOA's Henry Ridgwell in Manchester
The prime minister said the national terror threat level is being raised from severe to critical, which means another attack is expected "imminently."
She said it is possible a "wider group of individuals" may have been responsible for Monday's Manchester bombing -- something British police are urgently trying to determine.
Police have identified the suicide bomber as 22-year-old Salman Abedi, but have given no other information about him.
Islamic State is claiming it was behind the attack, but neither British nor U.S. intelligence have confirmed that.
Police raided a residential area of Manchester Tuesday and carried out a controlled explosion at a house in Fallowfield, a racially mixed suburb.
They also arrested a 23-year-old man elsewhere in Manchester, but have not given any information about how he may have been involved in the blast.
The bomber blew himself up in a hall just outside the arena, where American pop star Grande had just ended her show.
May said the bomber deliberately chose the "time and place to cause maximum carnage and to kill and injure indiscriminately."
Video shows the joy in the audience turning to confusion and then to panic and a scramble to get out of the building as the realization of what just happened spread.
Witness say they saw blood covered bodies on the floor while others, badly wounded, staggered toward the exits.
Many of the victims were young girls who idolize Grande. The youngest was just 8 years old.
The scene outside the concert hall was just as chaotic, with traffic snarled and parents rushing to the scene. Some mothers and fathers were still looking for their children on Tuesday.
Grande wrote on Twitter that she is "Broken. From the bottom of my heart, I am so, so sorry. I don't have words."
Pictures of Grande posing at an earlier show with one of the victims have cropped up on social media.
Britain's Queen Elizabeth held a moment of silence at a garden party at Buckingham Palace. French President Emmauel Macron signed a condolence book at the British embassy in Paris. German Chancellor Angela Merkel said the attack only strengthens Germany's resolve to work with the British.
The U.N. Security Council also condemned the bombing.
President Donald Trump, visiting Bethlehem in the West Bank, called those responsible for the blast "evil losers in life."
WATCH: Trump's reaction to Manchester attack
"I won't call them monsters because they would like that term; they would think that's a great name," Trump said. "I will call them from now on losers, because that's what they are."
He added, "We cannot stand a moment longer for the slaughter of innocent people."
May said she does not want the British people to feel "unduly alarmed," but she appealed to them to cooperate with police and other security services.
She ended her address to the nation Tuesday night with the traditional appeal to Britons to carry on and stand defiant.
"The spirit of Manchester and the spirit of Britain is far mightier than the sick plots of depraved terrorists. That is why the terrorists will never win, and we will prevail."