Eight people, including a number of children, have been injured in a knife attack at a dance workshop with eyewitnesses describing the scene like a "horror movie".
Merseyside Police said officers were called to a "major incident" at a property on Hart Street in Southport at about 11:50 BST.
The force said armed police "detained a male and seized a knife" and there was "no wider threat to the public".
North West Ambulance Service (NWAS) said they had treated eight patients with stab injuries and some had been taken to Alder Hey Children's Hospital.
Journalist Tim Johnson of Eye on Southport, who arrived at the scene about 20 minutes after the police had been called, said a Taylor Swift Yoga and Dance Workshop aimed at primary school children aged six to 10 had been taking place nearby.
The organisers have been approached for comment but have not yet responded.
Mr Johnson said the incident happened at the Hope of Hart Children's Club, which is housed in a former warehouse building on a back street.
He said those hurt were children and he had seen one girl on a stretcher who was seriously injured.
"It was horrendous. I've never seen anything like it," he said.
"There were so many police cars, it was a mass of blue lights.
"I saw ambulance men and women in tears. People were in tears in the streets."
PA MediaThe ages of those injured are not yet known
Alder Hey Children's Hospital said it had declared a "major incident" and was working with other emergency services as their emergency department is currently "extremely busy".
They said parents should only bring their children in if it was urgent, adding all other appointments and services were running as normal.
NWAS said it "dispatched 13 ambulances along with specialised resources" to the incidents and had also taken patients with stab injuries to Aintree University Hospital and Southport and Formby Hospital.
Great North Air Ambulance Service (GNAAS) confirmed their Langwathby-based critical care team worked alongside North West Air Ambulance, Midlands Air Ambulance Charity and a number of other emergency services at the scene.
A spokesman for GNAAS said: "We delivered advanced emergency care to one patient before accompanying them to hospital by road."
PA MediaThe road is closed while emergency services deal with the incident
A local business owner, who was one of the people who called the police, said it had been like a "scene in a horror movie".
Colin Parry, owner of Masters Vehicle Body Repairs on Hart Street, said he believed six or seven "young girls" had been stabbed.
"It's like something from America, not like sunny Southport," he said.
He said neighbours in the community wanted to "get" the suspect before police arrived.
One neighbour, helped by another, took "about 10 girls to safety" in his house, Colin told BBC Radio 5 Live.
"The community was coming together, everyone was trying to help," he said.
"Everyone was trying to save the young kids."
He said another man, a builder, also helped some of the children leave the site of the attack.
A woman, from Liverpool, said she was on the main shopping street, Lord Street, in Southport when she realised something "horrendous and "absolutely bewildering" was happening.
"All the shops were shutting their doors and I was thinking something's going on," she said.
She said "terror" had set in, adding: "Things like this don't happen around here."
David JackAlder Hey Children's Hospital has declared a "major incident" at the trust
Prime Minister Sir Keir Starmer said the multiple stabbings were "horrendous and deeply shocking" as he thanked emergency services for their response.
Home Secretary Yvette Cooper said she was "deeply concerned" about the "very serious incident", while Southport MP Patrick Hurley added that he was "hoping for the best possible outcomes to the casualties affected".
Steve Rotheram, mayor of Liverpool City Region, has urged the public not to spread "unconfirmed speculation and false information".