It was one of two loud explosions some witnesses reported minutes after United States pop singer Ariana Grande finished a concert at England's Manchester Arena.
The arena holds a capacity of 21,000 people, and the attack appears to be Britain's deadliest since a 2005 assault on London's transport system killed 52.
The chief constable of the Greater Manchester Police, Ian Hopkins, says a man believed to have set off a bomb at the arena is among the dead.
"We have been treating this as a terrorist incident, and we believe, at this stage, the attack last night was conducted by one man. The priority is to establish whether he was acting alone or as part of a network. The attacker, I can confirm, died at the arena. We believe the attacker was carrying an improvised explosive device, which he detonated, causing this atrocity."
Desperate parents and friends used social media to search for loved ones after the attack, with images of happy looking teenagers posted next to pleas for help.
This man, Steve Crosby, lives next door to the arena and describes hearing what he calls "an extremely loud bang" before emergency crews arrived.
"I'd seen smoke coming from the top of the arena, which made me think, 'That's more than just a speaker blowing up,' which people were presuming it was. As soon as I looked out the window, there were people outside my house - apartment - and people were hugging each other, crying."
The blast rocked the foyer of the arena as thousands of young fans and parents streamed out after the show.
The US singer's fan base is made up largely of teenagers and pre-teens.
Along with the tragedy has come efforts by local people to help those affected.
One woman named Edina was using social media to rally locals to help those who may now be stranded in Manchester.
"I'm sharing pictures of missing people, people who need help, people who have nowhere to sleep, no food or water, people who are stranded in Manchester, to try and get the awareness out for people who are willing to help."
British prime minister Theresa May has chaired a meeting of the government's emergency Cobra committee following the explosion she describes as an "appalling terrorist attack."
Home Secretary Amber Rudd has described the attack as a barbaric act and urged people to remain alert.
"The public should remain alert but not alarmed. And if they have anything to report, they should approach the police. But I have two further things to add. The great city of Manchester has been affected by terrorism before. Its spirit was not bowed, its community continued. This time, it has been a particular attack on the most vulnerable in our society. Its intention was to sow fear. Its intention is to divide. But it will not succeed."
In Australia, Prime Minister Malcolm Turnbull has expressed solidarity and sympathy with the British people following the attack.
In parliamentary question time, he said the country's thoughts were with those affected.
"Australia's heartfelt sympathy and resolute solidarity is with the people of the United Kingdom. We stand with them today, as we always have and always will, steadfast allies in freedom's cause."
Opposition Leader Bill Shorten followed with a statement of his own.
"So today, I offer my prayers and support to the people going through this and a promise to lots of kids wondering about all of this that this is not the normal course of events and we will never accept it as the normal state of affairs."
Ariana Grande made her first comment after the bombing with a tweet, writing, "Broken. From the bottom of my heart, I am so, so sorry. I don't have words."
The Australian High Commission in London is making urgent inquiries into whether any Australians have been hurt in the attack.
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