Updated at 8.30 pm with arrest and deaths

Two children have been killed and nine more injured, six critically, in a knife attack at a dance class in northern England on Monday, police said.

A 17-year-old male has been arrested on suspicion of murder and attempted murder over the attack in Southport, near Liverpool in northwest England, which also left two adults with critical injuries, Merseyside Police said in a news conference.

Police chief Serena Kennedy said counter-terror police had offered their support, but that the attack was not being treated as terror-related, and that the motivation was unclear.

All of those injured had been stabbed, she added.

The suspect is in custody and "enquiries are ongoing to establish the motive for this tragic incident", police said.

The North West Ambulance Service (NWAS) said it treated 11 casualties with stab injuries at the scene, who were then taken by ambulance and helicopter to Alder Hey Children's Hospital, and other hospitals in the local area.

Alder Hey Children's Hospital said: "We can confirm that the trust has declared a major incident."

Local business owner Colin Parry, one of the people who called police, told the domestic Press Association news agency that he believed several "young girls" had been stabbed.

Bare Varathan, who owns a local shop, told PA he saw "seven to 10 kids" who were "injured, bleeding", adding that he saw they had been stabbed.

Prime Minister Keir Starmer called the incident "horrendous and deeply shocking," adding on X, formerly called Twitter, that "my thoughts are with all those affected."

'Can't imagine the pain' 

Police were called to a property that was hosting a Taylor Swift-themed dance class for young children shortly before midday (1100 GMT), said Kennedy.

Officers were "shocked to find that multiple people, many of whom were children, had been subjected to a ferocious attack", she said.

The suspect apparently walked in from the street and started to attack the children, while adults in the room tried to protect them, added Kennedy.

Armed officers detained the suspect and seized a knife.

"As a mum and nana, I can't begin to imagine the pain and suffering that the families of the victims are going through," said Kennedy.

The area where the incident took place is in a quiet neighbourhood of residential streets.

Some residents who were allowed to come out from the police cordon sealing it off looked shocked, according to an AFP reporter at the scene. 

One man who lives in an adjoining street, who did not wish to give his name, told AFP: "It's not what you expect to see in Southport."