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, 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, defenseless 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. Islamic State 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".(Source: Reuters)