A suicide bomber killed at least 22 people and wounded 59 at a packed concert hall in the English city of Manchester in what Prime Minister Theresa May called a sickening act targeting children and young people.
Islamic State (IS) militant group, now being driven from territories in Syria and Iraq by Western-backed armed forces, said it carried out the attack. Some experts discounted this, noting there was no evidence of direct involvement and that details of the IS claim - in two contradictory postings - contrasted with the British police account.
Police announced a 23-year-old man had been arrested in connection with the attack, carried out late on Monday evening as people began leaving a concert given by Ariana Grande, a U.S. singer who attracts a large number of young and teenage fans.
In a statement made outside her Downing Street offices after a meeting with security and intelligence chiefs, May said police believed they knew the identity of the bomber.
"All acts of terrorism are cowardly," she said. "But this attack stands out for its appalling sickening cowardice, deliberately targeting innocent, defenceless children and young people who should have been enjoying one of the most memorable nights of their lives."
The attack came less than three weeks before a national election.
The northern English city remained on high alert, with additional armed police drafted in. London Mayor Sadiq Khan said more police had been ordered onto the streets of the British capital.
Monday's attack was the deadliest in Britain since four British Muslims killed 52 people in suicide bombings on London's transport system in 2005. But it will have reverberations far beyond British shores.
Attacks in cities including Paris, Nice, Brussels, St Petersburg, Berlin and London have shocked Europeans already anxious over security challenges from mass immigration and pockets of domestic Islamist radicalism. IS has repeatedly called for attacks as retaliation for Western involvement in the conflicts in Syria and Iraq.
Witnesses related the horror of the Manchester blast, which unleashed a stampede just as the concert ended at what is Europe's largest indoor arena, full to a capacity of 21,000.
"We ran and people were screaming around us and pushing on the stairs to go outside and people were falling down, girls were crying, and we saw these women being treated by paramedics having open wounds on their legs ... it was just chaos," said Sebastian Diaz, 19. "It was literally just a minute after it ended, the lights came on and the bomb went off."
A source with knowledge of the situation said the bomber's explosives were packed with metal and bolts.
A video posted on Twitter showed fans, many of them young, screaming and running from the venue. Dozens of parents frantically searched for their children, posting photos and pleading for information on social media.
"We were making our way out and when we were right by the door there was a massive explosion and everybody was screaming," Catherine Macfarlane told Reuters.
"It was a huge explosion - you could feel it in your chest."
World leaders expressed solidarity with Britain.
U.S. President Donald Trump spoke with May by telephone and agreed the attack was "particularly wanton and depraved", the White House said.
German Chancellor Angela Merkel said it "will only strengthen our resolve to...work with our British friends against those who plan and carry out such inhumane deeds".
Singer Ariana Grande, 23, said on Twitter: "broken. from the bottom of my heart, i am so so sorry. i don't have words."
May, who faces an election on June 8, said her thoughts were with the victims and their families. She and Jeremy Corbyn, the leader of the opposition Labour Party, agreed to suspend campaigning ahead of the election.
IS, while claiming responsibility on its Telegram account, appeared to contradict British police's description of a suicide bomber. It suggested explosive devices were placed "in the midst of the gatherings of the Crusaders".
"What comes next will be more severe on the worshippers of the cross," the Telegram posting said.
It appeared also to contradict in part a posting on another IS account, Amaq, which spoke of "a group of attackers". That reference, however, was later removed.
"It clearly bears the hallmark of Daesh (IS)," said former French intelligence agent Claude Moniquet, now a Brussels-based security consultant, "because Ariana Grande is a young singer who attracts a very young audience, teenagers.
"So very clearly the aim was to do as much harm as possible, to shock British society as much as possible."
IS supporters took to social media to celebrate the blast and some encouraged similar attacks elsewhere.
In March, a British-born convert to Islam ploughed a car into pedestrians on London's Westminster Bridge, killing four people before stabbing to death a police officer who was on the grounds of parliament. The man was shot dead at the scene.
In 2015, Pakistani student Abid Naseer was convicted in a U.S. court of conspiring with al Qaeda to blow up the Arndale shopping centre in the centre of Manchester in April 2009.
Desperate parents and friends used social media to search for loved ones who attended Monday's concert while the wounded were being treated at six hospitals across Manchester.
"Everyone pls share this, my little sister Emma was at the Ari concert tonight in #Manchester and she isn't answering her phone, pls help me," said one message posted alongside a picture of a blonde girl with flowers in her hair.
Paula Robinson, 48, from West Dalton about 40 miles east of Manchester, said she was at the train station next to the arena with her husband when she felt the explosion and saw dozens of teenage girls screaming and running away from arena.
"We ran out," Robinson told Reuters. "It was literally seconds after the explosion. I got the teens to run with me."
Robinson took dozens of teenage girls to the nearby Holiday Inn Express hotel and tweeted out her phone number to worried parents, telling them to meet her there. She said her phone had not stopped ringing since her tweet.
"Parents were frantic running about trying to get to their children. There were lots of lots of children at the Holiday Inn."