Twenty-two people have been killed and 59 injured in what Theresa May called an “appalling, sickening, terrorist attack” at Manchester Arena.
The attack happened in the foyer at 22:33 BST on Monday at the end of a concert by US singer Ariana Grande when a lone male set off a homemade bomb.
Relatives are using social media to hunt forloved ones, and an emergency number, 0161 856 9400, has been set up.
The first victim has been named as student Georgina Callander.
She was studying health and social care at Runshaw College in Lancashire.
In a statement in Downing Street, the prime minister said it was “now beyond doubt that the people of Manchester and of this country have fallen victim to a callous terrorist attack” that targeted “defenceless young people”.
She said the security services believe they know the attacker’s identity but are not yet able to confirm it.
Mrs May has chaired a meeting of the government’s emergency Cobra committee and is expected to travel to Manchester later.
Chief Constable Ian Hopkins said it was “the most horrific incident” the city had ever faced.
He said the “fast-moving investigation” was now working to establish whether the attacker “was acting alone or as part of a network”.
Sixty ambulances attended the incident and those wounded are being treated at eight hospitals around the city.Andy Holey, who had gone to the arena to pick up his wife and daughter, said: “An explosion went off and it threw me about 30ft from one set of doors to the other set of doors.
“When I got up I saw bodies lying on the ground. My first thought was to go into the arena to try to find my family.”Emma Johnson said she and her husband were at the arena to pick up her children, aged 15 and 17.
“We were stood at the top of the stairs and the glass exploded – it was near to where they were selling the merchandise,” she told BBC Radio Manchester.
“The whole building shook. There was a blast and then a flash of fire afterwards. There were bodies everywhere.”Teenager Abigail Walker, who was at the concert, told the BBC: “I had to make sure I had my sister. I grabbed hold of her and pulled hard. Everyone was running and crying.
“We were just trying to figure where everyone was. It was absolutely terrifying.”Charlotte Campbell’s daughter Olivia has been missing since the concert.
“She’s only a 15-year-old girl, she’s out there on her own because her friend has been found,” she told the BBC.
“If anyone sees her contact me. Give her your phone and let her ring me. I just want her home.Police have established a help centre at Manchester City’s Etihad Stadium, access Gate 11, for anyone who needs assistance in tracing loved ones.
Twitter has been flooded with appeals from relatives and friends of missing concertgoers via the hashtag #MissingInManchester.
Facebook also activated a safety check feature so that people can let their family and friends know they are safe.The blast happened close to the entrance to Victoria railway and tram station. The station has been closed and all trains cancelled.
Police also carried out a precautionary controlled explosion in the Cathedral Garden area of the city at about 01:32. The force later confirmed it was not a dangerous item.
Greater Manchester Mayor Andy Burnham said the city would “pull together”, adding: “That’s what we are. That’s what we do. They won’t win.”
The Manchester Arena or MEN is the city’s largest indoor venue with a concert capacity of around 21,000.
Police are encouraging anyone with footage from the scene to upload it atukpoliceimageappeal.co.uk or ukpoliceimageappeal.com. Other information can be reported to the anti-terrorist hotline on 0800 789 321.