Two children have died and nine were injured, six critically, after a knifeman launched a "ferocious attack" at a children's dance workshop.
Two adults are also in a critical condition after being stabbed as they tried to protect children at the Taylor Swift-themed event on Hart Street in Southport, Merseyside Police said.
A 17-year-old boy, from Banks in Lancashire, has been arrested on suspicion of murder and attempted murder.
Police said the motivation for the attack was "unclear" but it was not being treated as terror-related. One witness described the scene as "horrendous" and said she had "never seen anything like it".
Merseyside Police Chief Constable Serena Kennedy told a news conference officers were called to the scene following reports of a stabbing at 11:47 BST.
She said 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.
"It is understood that 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," she said.
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