A total of 27 people were taken to hospital by ambulance after Monday evening’s incident on Water Street in the city centre.
Four of them were trapped under the vehicle, with fire crews moving “rapidly” to free them so they could be treated.
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Twenty others were treated at the scene for minor injuries, and a cyclist paramedic 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 from the Liverpool area 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.
Footage shows car drive into crowd at speed
ACC Sims described the collision, which unfolded as the parade was winding down, as a “terrible tragedy” – with detectives declaring a major incident.
She added it is “vital people do not speculate or spread misinformation on social media” – and urged the public to refrain from sharing distressing content.
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.
Liverpool parade collision: What we know so far
Sir Keir Starmer described the scenes as “appalling” – and praised “the remarkable bravery shown by the police and other emergency services”.
The prime minister said: “Everyone, especially children, should be able to celebrate their heroes without this horror.
“The city has a long and proud history of coming together through difficult times.
“Liverpool stands together and the whole country stands with Liverpool.”
Eyewitness Natasha Rinaldi said she was watching the parade from her friend’s living room window when “we heard some screams”.
She added: “People started rushing to go after the driver and they tried to break the car. The police did everything to block and to push people away.”
Harry Rashid – who was attending with his wife and two young daughters – said the people carrier was “extremely fast”.
The 48-year-old heard people being knocked off the bonnet of a car. He said the car stalled for 10 seconds before “he put his foot down again and just ploughed through the rest of them”.
“It was horrible. My daughter started screaming and there were people on the ground,” Mr Rashid said.
