By Phil Noble and Sam Tobin

LIVERPOOL, England (Reuters) -A car ploughed into a crowd of Liverpool fans during a parade celebrating their side's Premier League soccer title on Monday, hospitalising 27 people, with two seriously injured, but police said they did not believe the incident was terrorism-related.

Police said they had arrested a "53-year-old white British man from the Liverpool area," whom they believed to be the driver of the vehicle which struck a large group of supporters who were celebrating in the city in northwest England.

Twenty people were treated at the scene. Ambulance officials said of the 27 taken to hospital, four were children. One child and one adult were in a serious condition. Four people trapped under the vehicle had to be released by fire fighters.

Videos on social media showed people thrown into the air as the car rammed into spectators.

When the car stopped, angry fans converged on it and began smashing the windows as police officers intervened to prevent them from reaching the driver.

"We believe this to be an isolated incident, and we are not currently looking for anyone else in relation to it. The incident is not being treated as terrorism," temporary Deputy Chief Constable Jenny Sims told reporters.

With most people off work for the Spring Bank Holiday, hundreds of thousands of fans gathered to watch the Liverpool team and its staff travel through the city centre on an open-top bus with the Premier League trophy.

The incident "cast a very dark shadow over what had been a joyous day," Liverpool city council leader Liam Robinson said on social media.

In the aftermath, a Reuters photographer saw emergency services carrying victims on stretchers to ambulances and debris scattered on the road.

Police were unusually quick to give a description of the man they arrested.

Dal Babu, a former chief superintendent in London's Metropolitan Police, told the BBC this was an effort to cool social media speculation that the episode was an Islamist attack.

The same police force oversaw the response to the murder of three young girls in the nearby town of Southport last year, an incident which sparked days of rioting, sparked by speculation online over the identity of the attacker.

An eyewitness to Monday's incident who gave her name as Chelsea told BBC Radio that people packed onto the street were only alerted to the danger by screams from the crowd. That enabled some to jump out of the way as the driver showed no sign of slowing.

"With the commotion, that was the only reason we looked up, and thankfully, looked up and managed to jump out (of) the way in time," the woman said.

A Reuters witness said that before the incident, there was disorder in the city centre where the parade was due to pass, with overcrowding and spectators confused by a lack of signage about street closures or where they should go.

Liverpool last won the trophy during the COVID pandemic when celebrations were not permitted due to lockdowns.

Politicians in Britain and in Ireland, where the club is popular, thanked emergency services.

"My thoughts are with all those injured or affected," Prime Minister Keir Starmer said on X, calling the scenes "appalling." and saying that he was being updated about the events.

The team said on X it was in direct contact with police. "Our thoughts and prayers are with those who have been affected by this serious incident," Liverpool FC said.

(Reporting by Phil Noble, Sam Tobin and Muvija M; writing by Kate Holton; Editing by Hugh Lawson, Christian Radnedge, Cynthia Osterman and Leslie Adler)