Serious questions will be asked of English authorities after dozens were injured when a car ploughed into a crowd of supporters celebrating Liverpool’s Premier League title win.

Footage showed people being thrown into the air as the car struck spectators before fans, who had gathered in Liverpool’s city centre for an open-top bus parade, swarmed the vehicle when it came to a stop just after 6pm on Monday (local time).

WATCH THE VIDEO ABOVE: Shocking images as car driven through crowd of Liverpool supporters.

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A 53-year-old British man was detained by police at the scene and authorities believe it was an “isolated incident”.

They are not treating it as an act of terrorism.

“Honestly, it doesn’t feel real,” Australian witness Joey Griffin told Sunrise.

“It went from being the best day ever to no one was speaking. People were crying. It was horrible.”

An estimated 500,000 fans had lined the streets to watch their team move through the 16km route.

The chaos erupted on Water Street, about two kilometres from the parade’s endpoint.

Four children were among 27 people taken to hospital. A further 20 people were treated at the scene.

Four people trapped under the car, including a child, were rescued by firefighters.

Liverpool City Region Mayor Steve Rotheram told the BBC there was “detailed work” underway to find out how the car was able to enter a street that had been closed off for celebrations.

International security expert Will Geddes said there were many questions, including why it happened and how the driver was in the fan area.

“How did the vehicle get in there because certainly there have been hostile vehicle mitigation measures implemented in most cities across the United Kingdom since 2017 when we got a spate of attacks where vehicles were used, particularly in terrorism attacks,” he told Sunrise.

Geddes said he had reviewed footage of the incident and believed there had been a “serious failure” by authorities to keep people safe.

“We have some major events that go on in the United Kingdom very regularly and again the security measures which were put in place can never assure 100 per cent safety and security but they will certainly make it very, very difficult for someone with malintent to go in and carry out an attack, and especially a vehicle which we’ve seen ... being used as a delivery platform for an attack,” he said.

“So yes, it was a serious failure, particularly by Liverpool Council and by the police ...”

The section of street where the incident occurred remained cordoned off overnight, and significant debris remained as the city awoke on Tuesday.

“The scenes in Liverpool are appalling — my thoughts are with all those injured or affected,” Prime Minister Keir Starmer said.

“Everyone, especially children, should be able to celebrate their heroes without this horror.

“Liverpool stands together and the whole country stands with Liverpool.”