Two children are dead and nine other children were injured after a suspected teenage knifeman attacked a Taylor Swift-themed holiday dance class in England.
Of those nine, six children are in a critical condition. Two adults were also critically injured while trying to protect the children.
The 17-year-old suspect, born in Cardiff, is being held on suspicion of murder and attempted murder at the property in Hart Street, Southport, Merseyside, police said.
Police said they are not treating the incident as terror-related but the motive "remains unclear".
The suspect is from Banks, a village outside Southport, with police now stationed outside an address.
Merseyside Police had declared a major incident at the scene in Southport, after receiving emergency calls just before midday, with armed response vehicles, 13 ambulances and the fire service descending on the mass casualty scene, a school holiday club for children aged six to 11.
One local said one of the two adults injured in the attack locked herself in a room with children to protect them.
Merseyside Police Chief Constable Serena Kennedy, giving a statement to media in Southport this evening, said officers were called to a "ferocious" attack.
"When they arrived, they were shocked to find that multiple people, many of whom were children, had been subjected to a ferocious attack and had suffered serious injuries," she said.
"It is understood the children were attending a Taylor Swift event at a dance school when the offender armed with a knife walked into the premises and started to attack the children inside.
"As a mum of two daughters, and the nanna of a five-year-old grand-daughter, I cannot begin to imagine the pain and suffering the families of the victims are currently going through, and I want to send them our heartfelt condolences and sympathies."
All the casualties are now being treated in hospitals, including children's hospitals in Liverpool and Manchester.
Emergency services described a "horrific" and "chaotic" scene as they arrived at the children's club.
Colin Parry, owner of Masters Vehicle Body Repairs, near the scene, said it was "like a scene from a horror movie".
A large police cordon has been put up around the building in Hart Road, with police officers guarding either end.
British Prime Minister Keir Starmer said today's events were "truly awful".
"I know the whole country is deeply shocked at what they have seen and what they have heard," he said.
"I know I speak for everybody in the whole country in saying our thoughts and condolences are with the victims, their families, their friends and with the wider community.
"It is impossible to imagine what they are going through right now."
Taoiseach Simon Harris said he was "absolutely horrified at the devastating attack," adding that he is "sending the support of the Irish people to Keir Starmer, the families and communities".
