Young children are said to be among at least eight people stabbed in scenes one witness described as like a "horror movie".
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Armed police arrested a man and seized a knife after being called to the "major incident" at Meols Cop High School on Hart Street, in Southport, north of Liverpool in the UK, at about 11.50am local time on Monday.
North West Ambulance Service (NWAS) said it had so far treated eight patients with stab injuries, who were taken to nearby hospitals.
Home Secretary Yvette Cooper said she was "deeply concerned", as Alder Hey Hospital declared a major incident and asked parents not to bring their children to the emergency department unless it was urgent.
Colin Parry, owner of Masters Vehicle Body Repairs on Hart Street, who called the police, said he believed six or seven "young girls" had been stabbed.
He told the PA news agency: "The mothers are coming here now and screaming. It is like a scene from a horror movie."
"Police have got him," he added.
"It's like something from America, not like sunny Southport."
A local parent, who did not wish to be named, said his daughter was "traumatised" by the attack.
He said: "My daughter was in it and she was traumatised.
"She ran away and she's safe."
Merseyside Police said: "There are a number of reported casualties and more details will be confirmed when possible.
"Armed police have detained a male and seized a knife. He has been taken to a police station.
"Please avoid the area while we deal with this incident. There is no wider threat to the public."
Ms Cooper posted on X: "I am deeply concerned at the very serious incident in Southport. All my thoughts are with the families and loved ones of those affected."
Southport MP Patrick Hurley said he was "deeply concerned" by the reports.
"I am hoping for the best possible outcomes to the casualties affected," he wrote in a statement on X.
NWAS said it had dispatched 13 ambulances along with specialised resources of its Hazardous Area Response Team (Hart), an air ambulance and doctors.
with AP
Australian Associated Press