Children were among at least 22 people killed and 59 injured when a suicide bomber struck as fans streamed out of a concert by U.S. singer Ariana Grande in the English city of Manchester on Monday night.
According to Reuters, Prime Minister Theresa May called an emergency meeting with intelligence chiefs on the deadliest militant assault in Britain since four British Muslims killed 52 people in suicide bombings on London's transport system in July 2005.
Witnesses related the horror of the blast which prompted a stampede just as the concert ended at Europe's largest indoor arena.
"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," Diaz said.
U.S. President Donald Trump described the attack as the work of "evil losers." German Chancellor Angela Merkel said it "will only strengthen our resolve to...work together with our British friends against those who plan and carry out such inhumane deeds."
Police said the attacker detonated the explosives shortly after 10:33 pm at Manchester Arena, which has the capacity to hold 21,000 people. Children were among the dead, police said.
"We believe, at this stage, the attack last night was conducted by one man," Manchester Chief Constable Ian Hopkins told reporters. "The priority is to establish whether he was acting alone or as part of a network.
He declined to answer questions on the bomber's identity.
London Mayor Sadiq Khan said more police had been ordered onto the streets of the British capital. London's Victoria coach station was briefly closed after discovery of a suspect package.
A source with knowledge of the situation said the bomber's explosives were packed with metal and bolts. At least 19 of those wounded were in a critical condition, the source said.
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," concert-goer Catherine Macfarlane told Reuters.
"It was a huge explosion - you could feel it in your chest."
Ariana Grande, 23, whose concerts attract a very large proportion of young women and girls, 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 in two-and-a-half weeks, 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 June 8 election.
"We are working to establish the full details of what is being treated by the police as an appalling terrorist attack," May said in a statement.
There was no immediate claim of responsibility, but U.S. officials drew parallels to the coordinated attacks in November 2015 by Islamist militants on the Bataclan concert hall and other sites in Paris, which claimed about 130 lives.
High profile attacks, including in Paris, Nice, Brussels, St Petersburg, Berlin and London, have shocked Europeans who are already anxious over security challenges from mass immigration, open borders and pockets of Islamist radicalism.
"It clearly bears the hallmark of Daesh (Islamic State)," 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."
Islamic State supporters took to social media to celebrate the blast and some encouraged similar attacks elsewhere.
Britain is on its second-highest alert level of "severe," meaning an attack by militants is considered highly likely.
British counter-terrorism police have said they are making on average an arrest every day in connection with suspected terrorism.
In March, a British-born convert to Islam plowed 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 center in the center 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," she said. "There were lots of lots children at Holiday Inn."
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