22 people have died, including several children, after an explosion tore through people leaving Manchester Arena after a gig.
More than 100 people were left injured after an attacker detonated an improvised explosive at the venue, where revellers had been attending a concert by US singer Ariana Grande.
Pictures on social media showed chaos as fans fled in fear following the event. There were also postings showing the aftermath of the detonation site, with bodies of the dead lying prone on the ground.
Greater Manchester Police chief constable Ian Hopkins called it "the most horrific incident we've had to face in Greater Manchester."
In a statement, he said the thoughts of the force were with the victims, the injured people, and their families, and how they would do everything possible to support them.
Hopkins said there was a "fast-moving investigation" in place, and insisted Greater Manchester Police would continue to work with UK intelligence and counter-terrorism services to establish more details about the perpetrator.
Ariana Grande has said she is "broken" on Twitter in the wake of the tragedy, while other politicians and celebrities continue to express their sorrow and support on the social network.