Twenty-two people, including children, have been killed and 59 injured in a suicide attack at Manchester Arena.
The blast happened at 22:35 BST on Monday at the end of a 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.
Sixty ambulances attended the incident and those wounded are now being treated at six hospitals around the city.
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-singer - has a strong following among teenage girls and children.
She tweeted: "broken. from the bottom of my heart, I am so so sorry. I don't have words."
Home Secretary Amber Rudd said it was "a barbaric attack, deliberately targeting some of the most vulnerable".