Twenty-two people, including children, have been killed and 59 injured in a suicide attack at Manchester Arena.
The blast happened in the foyer at 22:35 BST on Monday at the end of a concert by US singer Ariana Grande.
Police said the lone male attacker, who died in the blast, detonated an improvised explosive device.
Relatives are using social media to hunt for missing loved ones, and an emergency number, 0161 856 9400, has been set up.
Chief Constable Ian Hopkins said it was “the most horrific incident” Greater Manchester 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 now being treated at six hospitals around the city.
Eyewitnesses described seeing metal nuts and bolts among the debris and spoke about the fear and confusion that gripped the concert-goers.
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.”
The UK threat level has been has been judged to be severe for nearly three years – which means an attack is considered highly likely.
But in recent months the tempo of counter terrorist activity has been increasing with – on average – an arrest every day.
After the attack in Westminster by Khalid Masood in March, police and security officials have been warning that further attacks were almost inevitable.
But they also believed that those were more likely to be low-tech involving knives or vehicles. The fact that the Manchester attack involved explosives will worry them.
It may not have been at the level of complexity seen in Paris in 2015, when multiple attackers sent from Syria used guns and suicide belts, but it will still have required planning to make an improvised explosive device.
Police have established a help centre at the 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.
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 at ukpoliceimageappeal.co.uk or ukpoliceimageappeal.com. Other information can be reported to the anti-terrorist hotline on 0800 789 321.