Two children have died and six are in a critical condition following a “horror movie” knife attack at a Taylor Swift-themed yoga class.

Merseyside Police said a 17-year-old boy, from the village of Banks, was arrested on suspicion of murder and attempted murder following the “major incident” at a property in Hart Street, Southport, north of Liverpool, at about 11.50am on Monday.

The teenager, who is originally from Cardiff, remains in custody, police said.

Merseyside Police Chief Constable Serena Kennedy said as well as the two children who died, nine children were injured in the “ferocious” attack and two adults are also in a critical condition.

We believe the adults who were injured were bravely trying to protect the children who were being attacked

Merseyside Police Chief Constable Serena Kennedy

Ms Kennedy said the force believe the adults were injured while “bravely” trying to protect the children.

She told reporters: “My officers were called to reports of a stabbing at 11.47 this morning at an address in Southport.

“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 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.

“We believe the adults who were injured were bravely trying to protect the children who were being attacked.”

Ms Kennedy said the motivation for the attack is “unclear” but it is currently not being treated as terror-related.

Following the press conference, the Prime Minister offered his condolences to the victims of the attack in Southport, and their families and friends, saying “the whole country is deeply shocked”.

Witnesses described hearing “screaming” and seeing bleeding children running from the scene.

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Forensic officers on Hart Street in Southport, Merseyside (James Speakman/PA)

Colin Parry, owner of Masters Vehicle Body Repairs, said: “The mothers are coming here now and screaming.

“It is like a scene from a horror movie.”

North West Ambulance Service (NWAS) said victims were taken to Alder Hey Children’s Hospital, Aintree University Hospital and Southport and Formby hospital.

Alder Hey declared a major incident and asked parents not to bring their children to the emergency department unless it was urgent.

A large police cordon was put up around the building in Hart Road, with police officers guarding either end.

Officers and vehicles could be seen from a distance inside the cordon, including scenes-of-crime officers in white boiler suits.

Bare Varathan, 35, who owns a corner shop in Hart Street, said: “I saw seven to 10 kids outside the nursery. They were injured, bleeding. They were in the road, running from the nursery.

“They had been stabbed, here, here, here, everywhere. (Mr Varathan indicated the neck, back and chest area)

“They were all aged about 10.

“Only two police jeeps and ambulances arrived, then armed police came and took him out of the nursery.

“Somebody told me he had arrived in a taxi and had a knife.”

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Merseyside Police Chief Constable Serena Kennedy speaking to the media at Southport Community Fire Centre (James Speakman/PA)

One resident, who lives in Hart Street but did not want to be named, said he was at home about midday when a woman pulled up outside in her car, screaming.

He said: “She was screaming ‘my daughter’s been stabbed’.

“It was a mum with her 10-year-old daughter who had been stabbed.

“I helped to get her back up to where the police cars and ambulances were.

“She didn’t look great but my focus was on getting her back up the street to get help.”

A large and growing media presence with reporters, photographers and TV crews assembled at the police tape.

Dozens of police cars, vans, fire engines and ambulances were also assembled a short distance away at a local school.