Police say a man accused of deliberately crashing a car into pedestrians outside Melbourne's Flinders Street train station, sending 19 people to hospital and creating peak-hour chaos, has a history of assault, drug use and mental health issues.

Key points: A white 4WD drives through pedestrians before crashing into a tram stop

A white 4WD drives through pedestrians before crashing into a tram stop Nineteen people hurt, four in a critical condition

Nineteen people hurt, four in a critical condition Police arrest two men, including the 32yo driver of the car

Police said the driver was a 32-year-old Australian citizen of Afghan descent, and was known to police, but was not on bail at the time of the incident.

Of the 19 admitted to hospital, 15 are in a stable condition — including a four-year-old boy and the driver — and four are in a critical condition.

Acting Chief Commissioner Shane Patton said the man was arrested by an "heroic" off-duty police officer.

"To step in when you see that trauma, not understanding what the threat is … and take somebody into custody not knowing what he is risking himself — it is heroic," he said.

But he said there was no evidence to indicate there was a connection with terrorism.

"We don't at this time have any evidence or any intelligence to indicate there's a connection with terrorism," he said.

However, he said they would continue to work with the Counter-terrorism Command to ensure there was no connection and ongoing threat.

A second man, 24, was seen filming the incident and was found to be carrying a bag with knifes in it.

He was also arrested, but police believe he had no links to the incident. He is still assisting police with their inquiries.

'Deliberate act'

"We believe based on what we have seen that it is a deliberate act. The motivations are unknown," Victoria Police Commander Russell Barrett said.

A witness, John, described seeing an SUV travelling "at high speed" and "hitting people" just after 4:30pm.

"I saw a car, a SUV coming at high speed and really just heard the collision with people with bags and what must be shopping trolleys — and I hope not prams," he told ABC Radio Melbourne.

A witness, John, described seeing an SUV travelling "at high speed" and "hitting people". ( Twitter: @sirVIX_a_lot )

"I've really never seen anything like this before and I haven't stopped shaking."

The injured include a preschool-aged child with a head injury, who was taken to the Royal Children's Hospital in a serious condition.

The owner of a bakery on Flinders Street, identified as Jim, said the intersection was "full of pedestrians" when the car drove through it.

"He came to rest just next to the tram stop — maybe the side of the tram stop stopped him — but the only thing that seemed to be slowing him down was the amount of pedestrians he had hit.

"All you could hear was just 'bang bang bang bang bang'."

Sorry, this video has expired 'This is a terrible, evil, cowardly act'

Premier Daniel Andrews described the incident as "an act of evil and an act of cowardice".

"We have seen an horrific act … perpetrated against innocent bystanders," he said.

"But at the same time we have seen in the worst of times extraordinary courage, bravery and that Victorian spirit where people unknown to each other have extended a hand of care and friendship and love."

A number of pedestrians have been hit by a 4WD near Flinders Street station in central Melbourne. ( ABC News )

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 1 m More than a dozen people were hurt when the car ran into pedestrians

'There was blood and people crying'

Another witness, David, said the car appeared to be travelling between 60 and 100 kilometres per hour.

"So it hit people in a fair speed and at least one or two of them got thrown pretty high into the air," he said.

"When the police arrived, which was probably after about two to three minutes at the most, they arrested the driver."

Sarah, 18, and a friend said they got off a train at Flinders Street Station and saw people crying.

"It was scary because we didn't know what was going on," Sarah said.

Sorry, this video has expired Witness Jim describes how the Flinders Street Station incident unfolded

Police said fourteen people were injured after a car hit pedestrians on Flinders St. ( ABC News: Gus Goswell )

She said the scene outside the station was "horrific" and the injuries looked "very serious".

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"We saw like 10 pedestrians on the ground," Sarah said.

"There were three groups of people surrounding them and there was blood and people crying and it was just very, very sad."

Police said they "saturated the CBD area in the vicinity of Flinders, Elizabeth and Swanston streets to ensure community safety".

Counsellors were also sent to the scene.

Commander Barrett said the crime scene would remain in place for a considerable time and he urged people to avoid the area.

Paramedics attend the scene near Flinders Street Station in Melbourne. ( ABC News: Loretta Florance )

A 4WD ran into pedestrians outside Melbourne's Flinders Street station.

He said people who had left vehicles or items inside the crime scene would not be able to retrieve them until it reopened.

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"Police will continue to have a strong presence in the Melbourne CBD tonight," Commander Barrett said.

"We understand this has been a very traumatic event and there'll be witnesses, victims and family members affected."

Melbourne has been on high alert for a pedestrian attack since a driver allegedly ran down pedestrians in the busy Bourke Street mall in January.

Temporary concrete bollards were subsequently placed around key CBD locations, including Southern Cross Station and Federation Square, to prevent vehicle attacks.

The Victorian Government has since begun replacing them with permanent security features, such as planter boxes, as part of a $10 million CBD security upgrade.

Crowds form outside Flinders St Station. ( ABC News: Richard Willingham )

Police are appealing for any witnesses to attend the Melbourne West Police Station at 313 Spencer Street, Melbourne or contact Crime Stoppers on 1800 333 000.

They have also asked for anyone with video footage or images to upload them via the Police Evidence Submission Site.

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A police officer attends the scene of the incident near Flinders Street Station. ( ABC News: Loretta Florance )