British authorities believe they know the identity of the suicide bomber who killed 22 people including children at a pop concert, Prime Minister Theresa May said on Tuesday, as police said they had arrested a 23-year-old man in connection with the attack.
May said the "cowardice of the attacker was met by the bravery of the emergency services and the people of Manchester" in remarks given at Downing Street, her London office, following a meeting of the government's emergency committee.
"This was among the worst terrorist incidents we have ever experienced in the United Kingdom," she said.
"And although it is not the first time Manchester has suffered in this way, it is the worst attack the city has experienced and the worst-ever to hit the north of England."
Greater Manchester Police said they arrested a 23-year-old man suspected of links to the attack, while a second man was detained at the city's Arndale shopping centre but was "not currently believed to be connected to last night's attacks."
The shopping centre was briefly evacuated on Tuesday morning, reports said.
May said she planned to visit Manchester on Tuesday, while Andy Burnham, the city's mayor, said a vigil was planned for the victims in Manchester's central Albert Square at 6 pm (1700 GMT).
The White House said US President Donald Trump spoke to May by telephone to "offer condolences and support" on behalf of the United States.
Leaders from across the world sent condolences after the explosion late Monday, which came as thousands of people, many of them teenagers, were leaving Manchester Arena at the end of a concert by pop singer Ariana Grande.
"My thoughts and prayers are with the victims, their families and all those affected by this atrocity," Irish Prime Minister Enda Kenny said. "I am heartbroken for all."
"The city of Manchester has exceptionally close ties with our country and I extend the solidarity of the Irish government and all our people to those affected across Britain," Kenny said in a statement.
The police said they believed a lone male attacker died in the attack after he detonated a home-made device in the foyer of the Manchester Arena.
"We have been treating this as a terrorist incident and we believe at this stage the attack last night was conducted by one man," Greater Manchester Police Chief Constable Ian Hopkins said.
"The priority is to establish whether he was acting alone or as part of a network," Hopkins said.
"We believe the attacker was carrying an improvised explosive device which he detonated causing this atrocity."
At least 59 people were injured in the attack in the northern English city.
Pop star Grande, 23, said she was "broken" by the news of the deaths.
"From the bottom of my heart I am so, so sorry," she wrote on Twitter. "I don't have words."
Trump, on a visit to the Middle East, called those behind the attack "evil losers".
"I won't call them monsters, because they would like that term," said Trump, adding that so many "young, beautiful" people has lost their lives.
Australian Prime Minister Malcolm Turnbull also remarked how young many of the victims could be.
"This attack is especially vile, especially criminal, especially horrific - because it appears to have been deliberately directed at teenagers," he said in a statement.
Italy's government asked all the schools in the country to observe a minute's silence at noon (1000 GMT) to commemorate the victims of the attack.
Paris, the site of numerous terrorist attacks in recent years, will again turn off the lights of the Eiffel Tower later as a mark of respect while flags were flying at half mast at many locations across Britain and Europe, including at the European Commission in Brussels
Europe has been rocked by a string of Islamist terrorist attacks in the last three years, with five people being killed in London in March after a man drove a car at pedestrians and then stabbed a police officer.
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