LONDON — At least eight people, some of them children, were injured in a stabbing attack in northwest England on Monday, emergency services said. Police said they detained a 17-year-old male suspect and seized a knife.

A witness described seeing bloodied children running from a community center where a Taylor Swift-themed dance and yoga event for children aged about 6 to 11 was taking place. An advertisement for the event promised “a morning of Taylor Swift-themed yoga, dance and bracelet making.”

Prime Minister Keir Starmer called the attack “horrendous and deeply shocking.”

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Merseyside Police said officers were called at about noon to an address in Southport, a seaside town of about 100,000 people near Liverpool. It called it a “major incident” but said there was no wider threat to the public.

Detectives were not treating the attack as terror-related, the force said.

The suspect, who has not been identified, lived about 5 miles (8 kilometers) from the site of the attack, police said.

The North West Ambulance Service said medics treated eight people with stab injuries. The injured were taken to local hospitals, including a children’s hospital.

Bare Varathan, who owns a shop down the street, said he saw between seven and 10 bleeding children running from the Hart Space, a community hub that offers classes for expectant mothers, babies and children.

“They had been stabbed here, here, here, everywhere,” he said, indicating the neck, back and chest. “They were all aged about 10. One of them was really seriously injured.”

Ryan Carney, who lives with his mother in the street, said his mother saw emergency workers carrying children “covered in red, covered in blood. She said she could see the stab wounds in the backs of the children.”

“All this stuff never really happens around here,” he said. “You hear of it, stabbings and stuff like that in major cities, your Manchesters, your Londons. This is sunny Southport. That’s what people call it. The sun’s out. It’s a lovely place to be.”