At least 19 people were killed and 59 more injured in a blast that occurred outside Manchester Arena right after the end of a concert late Monday night.
Fifty-nine people injured in the explosion were taken to hospitals, the North West Ambulance Service (NWAS) NHS Trust said on Tuesday.
"NWAS has taken 59 casualties from the Manchester Arena incident to various hospitals and treated a number of walking wounded on scene," service said in a Twitter message.
"This is currently being treated as a terrorist incident until police know otherwise," said the police in a statement.
They said the explosion struck on 10:30 p.m. local time (2130 GMT) as people were leaving a concert by the American pop star Ariana Grande.
Eye witnesses spoke of hearing an explosion which sparked panic, as adults and children, many in tears or screaming, ran for their lives from the danger.
One described it as a stampede, with people being trampled on. Workers with China's Consulate General in Manchester, who rushed to the scene after the explosion, told Xinhua over phone that the area had been cordoned off with a heavy presence of police vehicles in sight.
One man told local media how he had just been 15 meters from the blast and was thrown through a door by the force of the blast. When he got back onto his feet and turned around he saw bodies on the floor, many people either dead or unconscious. He said there were between 20 to 30 bodies.
Greater Manchester Police (GMP) declared a major incident and evacuated a wide area, as armed police stood by.
Many parents who had been waiting to collect their children close to the exits of the arena were given shelter in nearby hotels as they desperately awaited news.
Army bomb disposal units raced to the area to examine the scene and search for other devices. A controlled explosion was carried out on a second suspect device close by in a garden close to the city's cathedral.
Victoria Railway Station, just meters from the stadium, had been preparing for thousands of people making their way home. Instead the station was evacuated and trains halted, leaving thousands of people stranded.
Regional ambulance services raced to the scene to take casualties, many of them seriously injured, to hospitals.
A spokesman for the pop star said Ariana Grande was unhurt in the attack.
British Prime Minister Theresa May said the government is trying to establish "the full details of what is being treated by the police as an appalling terrorist attack."
The prime minister is to chair an emergency Cobra meeting on Tuesday following the blast. Downing Street said it is expected to take place at around 09:00 BST (0800 GMT).
Manchester City councillor Pat Karney told BBC Radio 5 live the arena was "a very easy target."
"A concert hall where young people are enjoying music -- and nobody would contemplate anything like this happening," he added.
"Everyone in the city is shocked, having seen how young some of these young people are, our thoughts are obviously with their families."
Manchester Arena, formerly known as the MEN Arena, managed and operated by SMG Europe, is one of the busiest venues in the world and the largest indoor arena in Europe.
The Arena is in Hunts Bank, Manchester, situated immediately north of the city center, and most of the arena is situated above Manchester Victoria station in air rights space.
The arena regularly hosts concerts by major stars like Ariana Grande -- a 23-year-old American TV teen actress-turned-pop star. She is currently on a European tour, and has already played Birmingham and Dublin with concerts due at O2 Arena in London on Wednesday and Thursday.
Cabs are offering free rides for the concertgoers to nearby hotels, and hundreds of people in the city are offering spare beds and rooms for those stranded in the city.
A large area of Manchester city center, just a few hundred meters from the arena, was destroyed in 1996 by a bomb planted by the Irish Republican Army.
