Two children have been killed and nine injured, six critically, in a "ferocious" knife attack at a children's dance workshop.
Two adults are also in a critical condition after being stabbed as they tried to protect children at the Taylor Swift-themed event on Hart Street in Southport, Merseyside Police said.
A 17-year-old boy, from Banks in Lancashire, has been arrested on suspicion of murder and attempted murder.
Police said the motivation for the attack was "unclear" but it was not being treated as terror-related.
One witness described the scene as "horrendous" and said they had "never seen anything like it".
The King and the prime minister have led tributes to the victims, offering their "heartfelt condolences" to those affected.
Merseyside Police declared a major incident after receiving emergency calls at 11.47 BST, on what was the first full week of the school summer holidays for many children in the UK.
Armed response vehicles, 13 ambulances and the fire service rushed to the dance class, which was being held for children aged six to 10.
Chief Constable Serena Kennedy told a news conference that officers responding to the calls "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 that 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," she said.
"We believe that the adults who were injured were bravely trying to protect the children who were being attacked."
She added: "As a mum of two daughters, and the nana of a five-year-old granddaughter, I cannot begin to imagine the pain and suffering the families of the victims are currently going through and I want to send them our heartfelt condolences and sympathies".
Ms Kennedy said the 17-year-old suspect, who police said was born in Cardiff, will now be questioned by detectives.
Merseyside Police said it was not looking for anyone else in connection with the attack and that the "motivation for the incident remains unclear".
Ms Kennedy added that Counter Terrorism Police North West had offered their support to Merseyside Police but that the incident was not currently being treated as terror-related.
PA Media Police in Southport, Merseyside, where a man has been detained and a knife has been seized after a number of people were injured in a reported stabbing
Journalist Tim Johnson, from Eye on Southport, said the attack happened at the Hope of Hart children's club, which is housed in a former warehouse building on a back street.
"It was horrendous. I've never seen anything like it," Mr Johnson 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."
Alder Hey Children's Hospital declared a "major incident", while the North West Ambulance Service said it had sent 13 ambulances to the scene.
Dave Kitchin, head of operations at the ambulance service, said they treated 11 casualties at the scene, who were sent by emergency ambulance and heli-med to Alder Hey and Royal Manchester Children’s hospitals, Aintree University Hospital, Southport and Formby District General Hospital and Ormskirk District General Hospital.
He described the scene that met paramedics as "devastating", adding, "no doubt this incident will have a lasting impact on the whole community, and our thoughts are very much with them at this difficult time".
Great North Air Ambulance Service confirmed its critical care team was also sent to the scene.
A spokesman added: "We delivered advanced emergency care to one patient before accompanying them to hospital by road."
Colin Parry, owner of Masters Vehicle Body Repairs, which is next door to where the attack happened, said that shortly before it began there was a commotion outside because a young man wearing a green hoodie and a face mask had arrived by taxi but was refusing to pay the driver.
He said an employee called him back out a short time later and that he saw numerous "young kids, all bleeding".
"It's like something from America, not like sunny Southport."
Mr Parry told BBC Radio 5 Live that a builder helped lead some of the children away from the scene of the attack and neighbours helped take "about 10 girls to safety".
"The community was coming together, everyone was trying to help. Everyone was trying to save the young kids," he added.
In a statement on X, The King said he and his wife were "profoundly shocked" to hear of the "utterly horrific incident".
He added: "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, and to all those affected by this truly appalling attack."
The Prince and Princess of Wales called the attack "horrid and heinous" adding that they were sending "love, thoughts and prayers to all those involved".
They said on X: "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.
"Thank you also to the emergency responders who, despite being met with the most horrific scenes, demonstrated compassion and professionalism when your community needed you most."
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