Two people, one of them a child, have sustained serious injuries after a car ploughed into crowds at the Liverpool FC trophy parade.
A total of 27 people were taken to hospital by ambulance after Monday evening's incident on Water Street in the city centre.
Twenty others were treated at the scene for minor injuries, and a cyclist paramedic at the scene was among those struck by the vehicle.
At a news conference, Assistant Chief Constable Jenny Sims stressed that the incident is not being treated as terror-related.
She said a 53-year-old white British man remains in custody, who is believed to be the person driving the vehicle.
"What I can tell you is that we believe this to be an isolated incident, and we are not currently looking for anyone else in connection to it," she added.
Hundreds of thousands of people were lining the streets to celebrate Liverpool winning the Premier League.
Ms Sims described the collision, which unfolded as the parade was winding down, as a "terrible tragedy" - with detectives immediately declaring a major incident.
She added that it is "vital people do not speculate or spread misinformation on social media" - and urged the public to refrain from sharing distressing footage.
David Kitchin from the North West Ambulance Service said paramedics already had a "substantial presence" close to the parade route, meaning they were able to respond quickly.
The 27 who needed hospital treatment - four of them children - were taken to four nearby hospitals: Royal Liverpool, Alder Hey, Arrowe Park and Aintree.
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Crowds swarm car in Liverpool 0:50
Crowds swarm car in Liverpool
Prime Minister Sir Keir Starmer described the scenes as "appalling".
"My thoughts are with all those injured or affected. I want to thank the police and emergency services for their swift and ongoing response to this shocking incident," he said.
"I'm being kept updated on developments and ask that we give the police the space they need to investigate."
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Home Secretary Yvette Cooper is also being kept updated about the events.
In a post on X, she wrote: "Thank you to the police and emergency services for their swift response to the truly shocking and horrendous scenes in Liverpool this evening.
"Thinking of all those affected at this very difficult time. The police are investigating, and I'm being kept updated on developments."
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