UPDATE: Four people are in a critical condition after a speeding car ploughed through pedestrians at a busy Melbourne CBD intersection in what police believe was a deliberate act.

A pre-school aged child suffering serious head injuries is among the 19 injured pedestrians taken to various hospitals in Melbourne.

One witness said the 4WD was travelling at up to 100km/h as it hit peak hour crowds crossing Flinders St, 11 months after six people were killed in a similar incident four blocks away in Bourke St.

Camera Icon Police drag one man from the crashed Suzuki. Credit: 7 News

“The only thing that slowed him down was him hitting pedestrians,” witness Jim said.

“There was no braking, there was no slowing down, he went straight through the intersection.

“All you could hear was the sound of the car hitting people and screams.”

Another witness said between 40 and 60 people were crossing the road at the time.

Camera Icon Witnesses say the Suzuki ran a red light before hitting pedestrians. Credit: 7 News

The 32-year-old driver was arrested by an off-duty police officer at the scene and was taken to hospital after resisting arrest.

“At this stage we believe it is a deliberate act,” Victoria Police commander Russell Barrett told reporters. “The motivation is unknown.”

The driver is an Australian citizen of Afghan descent who is known to police following a 2010 minor assault matter, with a history of drug use and mental health issues.

Acting police chief commissioner Shane Patton said the man is on a mental health plan and receiving treatment for mental illness, but was not on bail or any corrections order.

Mr Patton said there is no evidence to suggest any links to terrorism, but investigations continue.

He said the man was alone in the car and accelerated as he drove on tram tracks down Flinders Street.

A second man, 24, was arrested after being seen filming the incident and being found with three knives in his bag, although police believe he has no links to the incident.

When the white Suzuki ended up crashing into a tram stop, around eight members of the public pulled the driver out of the car before police tackled and restrained him, according to witness accounts.

Paul Cahalane, 25, was walking to the Coles supermarket on Elizabeth Street when he heard a “bang”.

Camera Icon The damaged vehicle on Flinders Street. Credit: AAPIMAGE

“The first thing I saw was people sprinting in separate directions, saying ’bomb and this and that’,” he said.

“Straight away I saw about five or six people on the floor.”

He said one man appeared non-responsive and a woman had blood over her face.

A crime scene has been set up and will remain in place until 7am on Saturday.