At least 22 people, including children, have died after an explosion hit Manchester Arena at the end of a pop concert by US singer Ariana Grande.
A further 59 people were injured in the explosion which Greater Manchester Police believe was carried out by a single male attacker.
GMP's chief constable Ian Hopkins said a man detonated an "improvised explosive device" outside the Arena and died in the attack.
Witnesses reported hearing a "huge bang" at the venue shortly after the concert on Monday evening.
Chief constable Mr Hopkins said police believe the attacker acted alone but said the priority was to find out if he was part of a wider network.
"This is the most horrific incident we have ever faced," he said. "We have been treating this as a terrorist incident."
He added: "I can confirm children are among the deceased."
Network Rail said Manchester Victoria station, located near the venue, has been closed.
Footage on social media showed lines of ambulances and armed police in attendance.
One fan at the concert described how the explosion hit the venue as fans were leaving the building.
Majid Khan, 22, said: "We were all exiting the venue when around 10.40-10.45pm-ish a huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena.
"It was one bang and essentially everyone from the other side of the arena where the bang was heard from suddenly came running towards us as they were trying to exit.
"Everyone was just running to any exit they could find as quickly as they could.
"Everyone was in a huge state of panic, calling each other as some had gone to the toilet whilst this had gone off, so it was just extremely disturbing for everyone there."
Home Secretary Amber Rudd has described it as a "barbaric act", while Prime Minister Theresa May said her thoughts are with those affected by the "appalling" incident and will chair a Cobra meeting on Tuesday.
A spokesman for singer Ms Grande said she is uninjured.