Some of the dead were children, they said.
The 23-year-old star regularly attracts a young crowd to her concerts - her audiences are often filled with teenage girls experiencing such an event for the first time.
"There was an explosion heard as we were leaving the arena and then another one when we got outside", concert-goer Sibion Joyce, from Bury in Greater Manchester, told news.com.au.
"Parents were frantic running about trying to get to their children", she said.
In a statement, Manchester Police Chief Constable Ian Hopkins said the attack was "the most horrific incident" the city has ever had to face. A large majority of those in attendance at last night's concert were young families and teenagers.
Mark Harrison, who accompanied his 12-year-old daughter to the concert from Cumbria in northern England, said there weren't metal detectors or body checks at the arena's entrance, though bags were inspected and items such as water bottles had to be discarded.
May said police had identified the suspected attacker, though she did not reveal his name. "People started running and shouting that there was a bomb. People were getting shoved and pushed everywhere".
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In an interview with United States media, President Trump also accused him being a "showboat" and a "grandstander". She added that McGahn "got that it was serious" when they discussed Flynn on January 26 and again on January 27.
He said: "She was so young & sweet".
Friends of Georgina, also known as Gina, described her as "a handsome girl with the kindest heart and soul". The head teacher of the Tarleton Community Primary School that Saffie attended in the village of Tarleton, Lancashire, described her as "simply a lovely little girl in every aspect of the word". She said police had been informed but "obviously are swamped with calls". "We were getting ready to leave the arena and the next minute we heard a mighty bang coming from the left-hand side of the arena", Carole Long, who was at the concert with her 10-year-old daughter Robin, told CNN. "We believe the attacker was carrying an improvised explosive device which he detonated causing this atrocity".
Prime Minister Benjamin Netanyahu said his government "strongly condemns the bad terror attack".
"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".
On major Israeli websites Tuesday morning, the attack for a time overshadowed President Trump's visit to the country. I won't call them monsters because they would like that term. "I saw five people with blood all down them". "But they're losers. Just remember that".
"My thoughts and prayers are with those affected, and we stand by our nearest neighbour, the United Kingdom especially due to the strong links between our country and the city of Manchester", he said.
"The Chinese people are firmly standing together with the British people at this hard time", Xi said.
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Hunter Strickland (1-1), Derek Law and Josh Osich pitched in relief for the Giants, who won for the seventh time in eight games. Athletics 8, Red Sox 3: Mark Canha, Khris Davis, Chad Pinder and Jed Lowrie each hit a home run, powering Oakland over Boston.
She wrote: "From the bottom of my heart, I am so so sorry, I don't have words". Twitter and Facebook lit up with heartbreaking appeals for the missing.
Theresa May gave a statement shortly after 11am before she travelled from London to Manchester.
Numerous concertgoers were teenagers and families. "We mourn the lives of children and loved ones taken by this cowardly act". "It was just after she finished her final song".
After the attack in Westminster by Khalid Masood in March, police and security officials have been warning that further attacks were nearly inevitable. Police believe Masood acted alone and was motivated by Islamist extremism. Campaigning for Britain's June 8 national election was suspended.
Manchester has been targeted by terrorism before. The city was hit by a huge Irish Republican Army bomb in 1996 that leveled a swath of the city center.
Katz reported from London.
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Israelis have experienced firsthand terror, President Trump said, adding that Islamic State threatens Jewish institutions. The only credible ideas still require the creation of an independent and sovereign Palestinian state alongside Israel.
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