A 9-year old girl wounded in a stabbing attack at a Taylor Swift-themed dance and yoga class in northwestern England has died, bringing the death toll to three, as police questioned a 17-year-old suspect.

Merseyside Police said the other fatalities were girls aged 6 and 7.

Eight children and two adults remain in hospital after the attack in Southport. Both adults and five of the children are in critical condition.

Swift said she was "completely in shock" and still taking in "the horror" of the event.

"These were just little kids at a dance class," she wrote on Instagram.

"I am at a complete loss for how to ever convey my sympathies to these families."

A 17-year-old boy was arrested on suspicion of murder and attempted murder.

Local people left flowers and stuffed animals in tribute at a police cordon on the street lined with brick houses in the seaside resort near Liverpool — nicknamed "sunny Southport" — whose beach and pier attract vacationers from across northwest England.

Prime Minister and Royals respond to Southport stabbing. – Reuters

Witnesses described scenes "from a horror movie" as bloodied children ran from the attack just before noon on Monday. The suspect was arrested soon after on suspicion of murder and attempted murder. Police said he was born in Cardiff, Wales, and had lived for years in a village about 3 miles (5 kilometers) from Southport. He has not yet been charged.

Police said detectives are not treating Monday's attack as terror-related and they are not looking for any other suspects.

"We believe the adults who were injured were bravely trying to protect the children who were being attacked," Merseyside Police Chief Constable Serena Kennedy said.

It is the latest shocking attack in a country where a recent rise in knife crime has stoked anxieties and led to calls for the government to do more to clamp down on bladed weapons.

Witnesses described hearing screams and seeing children covered in blood emerging from the Hart Space, a community centre that hosts everything from pregnancy workshops and meditation sessions to women's boot camps.

"They were in the road, running from the nursery," said Bare Varathan, who owns a shop nearby. "They had been stabbed, here, here, here, everywhere," he said, indicating the neck, back and chest.

Police and emergency services at the site of a stabbing in Southport, Merseyside. – AP

An advertisement posted online described the event as a yoga and dance workshop for children aged between six and 11.

Merseyside Police said the motive for the attack was unclear but it was not believed to be terrorism-related and they were not looking for anyone else in connection with the stabbings.

Prime Minister Keir Starmer and King Charles both expressed their shock at the attack and sent their condolences to those affected.

"The events today are just truly awful, and I know the whole country is deeply shocked at what they've seen and what they've heard," Starmer said.

King Charles said that it was a "truly appalling attack".

"My wife and I have been profoundly shocked to hear of the utterly horrific incident in Southport today," Charles said in a statement.

"We send our most heartfelt condolences, prayers and deepest sympathies to the families and loved ones of those who have so tragically lost their lives."

Tributes near the scene of the stabbing incident in Southport, UK. – Reuters

Prince William and his wife Catherine said that "as parents, we cannot begin to imagine what the families, friends and loved ones of those killed and injured in Southport today are going through."

Colin Parry, who owns a nearby auto body shop, told The Guardian that the suspect arrived by taxi.

"He came down our driveway in a taxi and didn't pay for the taxi, so I confronted him at that point," Parry was quoted as saying. "He was quite aggressive, he said, 'What are you gonna do about it?'"

Parry said most of the victims appeared to be young girls.

"The mothers are coming here now and screaming," Parry said.

"It is like a scene from a horror movie. ... It's like something from America, not like sunny Southport."

Britain's worst attack on children occurred in 1996, when 43-year-old Thomas Hamilton shot 16 kindergarteners and their teacher dead in a school gymnasium in Dunblane, Scotland. The UK  subsequently banned the private ownership of almost all handguns.

Mass shootings and killings with firearms are rare in Britain, where knives were used in about 40 per cent of homicides in the year to March 2023.