Four children were among the 27 people taken to hospital after a vehicle ploughed into crowds gathered on Water Street in Liverpool city Centre.
Two victims, including a child, sustained serious injuries.
An additional 20 people were treated at the scene for minor injuries, according to authorities.
Merseyside Police confirmed the arrest of a 53-year-old white British man from the local area at the scene around 6 pm.
He is believed to have been the driver of the vehicle, BBC reported.
Police said they are not treating the incident as terrorism-related.
Footage circulating online shows the car veering into both sides of the packed street, apparently accelerating into the crowd.
Prime Minister Keir Starmer described the events as “appalling,” adding, “I’m being kept updated on developments and ask that we give the police the space they need to investigate.”
Emergency vehicles remained at the scene for hours, with police, ambulances, and fire engines stationed along Water Street.
The incident occurred during a celebratory parade marking Liverpool FC’s Premier League triumph.
Up to one million fans had lined the 8-kilometer route, which started at Allerton Maze and was set to end at the Strand.