Twenty-two people, including children, have been killed and 59 injured in a suspected suicide attack at Manchester Arena.
The blast happened at 22:35 BST on Monday at the end of a pop concert by the US singer Ariana Grande.
Greater Manchester Police said the lone male attacker, who died in the blast, was carrying an improvised explosive device which he detonated.
Relatives are using social media to hunt for missing loved ones.
Police have set up an emergency telephone number in response to the attack. It is: 0161 856 9400.
He said the "fast-moving investigation" was now working to establish whether the attacker "was acting alone or as part of a network".
Home Secretary Amber Rudd said it was "a barbaric attack, deliberately targeting some of the most vulnerable".
The prime minister is to chair a meeting of the government's emergency Cobra committee at around 09:00.
Political parties have suspended general election campaigning.
The explosion occurred shortly after Ariana Grande left the stage at the arena - the city's largest indoor venue with a concert capacity of around 21,000.
Grande - a 23-year-old American TV teen actress-turned-pop star - has a strong following among teenage girls and children.
The pop star tweeted: "broken. from the bottom of my heart, i am so so sorry. i don't have words."
The blast occurred close to the entrance to Victoria train 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. After the explosion, witnesses spoke about the fear and confusion that gripped those caught up in the events.
Those inside the arena described clothing and mobile phones strewn across the floor as people scrambled for the exits.
Andy Holey, who had gone to the arena to pick up his wife and daughter who had been at the concert, 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.
"I managed to find them eventually and they're OK."
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