Merseyside Police Assistant Chief Constable Jenny Sims said the Bank Holiday Monday incident wasn't  being treated as terrorism and was believed to be an isolated incident following the arrest of the 53-year-old man at the scene.
Police said the suspect was the driver the of the car, white, British and from the Liverpool area.
Four children were among those injured after a car ploughed into the crowd during Liverpool's victory parade through the city streets, with one child seriously hurt.
At a press conference Dave Kitchin from North West Ambulance Service said 27 people were taken to hospital and 20 people were treated at the scene, with four children among the injured.
Two of those taken to hospital, including one of the children, suffered serious injuries.
Nick Searle, Merseyside Fire and Rescue Service chief fire officer, said four people who were trapped under the car, including a child, were rescued by firefighters.
Les Winsper, 55, told The Guardian the incident unfolded in front of him.
"People started banging on the (car) windows, then they smashed the windows and he panicked and put his foot down.
"He then hit someone and that person has gone in the air and he's ploughed through the rest of them.
"I've never seen anything like it."
Prime Minister Keir Starmer issued a statement condemning the attack.
"The scenes in Liverpool are appalling - my thoughts are with all those injured or affected," he said.
Liverpool Football Club said it would "continue to offer our full support to the emergency services and local authorities who are dealing with this incident".
Footage circulating online appeared to show the vehicle speed up as it veered into pedestrians on both sides of the crowded street.
Officers surrounded the car as witnesses attempted to stop the driver, who was eventually detained by police. A huge police cordon is in place on Water Street where emergency vehicles line the road.
Emergency services were quickly at the scene and several casualties were taken away by ambulances.
A number of police officers could be seen lining the junction of Castle Street and Water Street and firefighters were also on scene.
Emergency vehicles including ambulances were allowed out of the cordon to leave the area.
Harry Rashid, witnessed the collision, saying "this grey people carrier just pulled up from the right and just rammed into all the people at the side of us".
"It was extremely fast. Initially, we just heard the pop, pop, pop of people just being knocked off the bonnet of a car."
Another witness said he saw a car "speeding up" before hitting pedestrians.
Mike Maddra told the PA news agency he thought he saw two people being hit, saying "it looked deliberate".
Liverpool sealed the title last month with four games to spare and ended the season with a 10-point lead over second-placed Arsenal.
They lifted the trophy after their 1-1 draw with Crystal Palace at Anfield on Sunday.
During the celebrations earlier on Monday, players and staff waved to the crowd from an open-top bus with the words 'Ours. Again.' emblazoned on the side.
with PA
