Police say the man who crashed his car into Melbourne pedestrians yesterday has blamed "the mistreatment of Muslims" for "some of his activities", but they still do not know what motivated him to drive through a busy crossing.

Authorities say there is no increased threat to the public after Saeed Noori, a man with a history of drug use and mental illness, allegedly crashed into people outside Flinders Street train station.

Noori is a 32-year-old Australian of Afghan descent.

He was released from hospital this afternoon and is now is police custody.

Police say crime department detectives will interview him once he has been medically cleared.

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 Emergency services outside Flinders Street Station after pedestrians hit by car

The incident just after 4:40pm yesterday left 18 people in hospital, including a four-year-old boy.

Four people have since been discharged, but 14 remain in hospital, with three in a critical condition including an 83-year-old man.

Nine of the victims are overseas nationals from countries including South Korea, China, Italy, India, Venezuela, Ireland and New Zealand.

Victoria Police Acting Chief Commissioner Shane Patton said the condition of the four-year-old boy, who was taken to the Royal Children's Hospital with a head injury, had improved and is now listed as stable.

Witnesses described people being "thrown into the air" as the car drove through the crossing. ( ABC News: Gus Goswell )

Police have held "preliminary discussions" with the driver.

"Last night investigators gained some access to him and were able to not conduct a formal interview, but had some preliminary discussions with him," Acting Commissioner Patton told ABC News Breakfast.

"During those discussions, he made some utterances … in respect to a number of matters.

"He spoke of dreams and voices, but also attributed some of his activities as well due to the mistreatment of Muslims.

"Now, obviously we are continuing to explore this entire incident and trying to ascertain what the motivations are.

"We haven't ruled anything in or out and our counter-terrorism command continues to obviously assist Crime Command in this investigation."

Sorry, this video has expired Acting Chief Commissioner Shane Patton said the driver made "utterances" about mistreatment of Muslims.

Acting Commissioner Patton said properties were searched last night in West Heidelberg and Oak Park, but no links to terrorism were found.

"We haven't found anything at all to indicate his linkage or involvement with any type of extremism with any terrorism organisation or anything of a terrorist nature," he said.

Acting Commissioner Patton said Noori was on a mental health plan and had missed an appointment on the day of the crash.

Police said the driver was a 32-year-old Australian citizen of Afghan descent. ( AAP: Joe Castro )

Prime Minister Malcolm Turnbull said the incident was "shocking".

"To be attacked like this, in the middle of one of our great cities, is a shocking event, a shocking crime," he said.

"Our consular officials are working hard to get in touch with their families and make sure they are aware of what has happened to their loved ones."

Sorry, this video has expired 'I didn't sleep last night, you just think about life,' says Flinders St witness

Mr Turnbull said the driver came to Australia as a refugee.

"He did not come with a people smuggler, he came through the normal refugee programs," he said.

Witnesses reported hearing loud "bangs" as the car ploughed into pedestrians, saying people were "thrown into the air" before the car crashed into a tram station and came to a stop.

Police said Noori was known to them, but was not on bail or under any corrections orders at the time of the incident.

A group of youths from the Ahmadiyya Muslim community gathered outside Flinders Street Station yesterday in support of those injured.

"As Australian Muslims, it is not just a moral but also a religious duty to condemn in the strongest possible terms [yesterday's] horrific and senseless act of violence," the imam of the Ahmadiyya Muslim Community of Victoria said in a statement.

"We express our deepest sympathy and pray for the full recovery of those injured in the incident.

"Our youth have gathered here today because they sympathise with those who are affected and stand united against extremism."

A part of the white 4WD after it crashed through the pedestrian crossing. ( ABC News: Gus Goswell )

'Heroic' off-duty officer in hospital

Acting Commissioner Patton said a "heroic" off-duty police officer, who arrested the driver, was having surgery today for an injured finger.

He is now in a stable condition.

"[It was a] great response, great actions with everyone who responded at the time, including all the police and emergency services," Acting Commissioner Patton said.

Police Association secretary Wayne Gatt said up to 70 officers were at the scene, including about 16 who were on their first day in the job.

"Imagine that, your first day in Victoria Police after training [and] being confronted with that carnage," he said.

"It's a credit to their training and a credit to their colleagues and professionalism that they worked and performed so well yesterday."

Police said they believed it was an isolated incident and assured the public they were doing "everything we can to keep the community safe".

They have promised there will be an increased police presence at major events in coming weeks.

Acting Commissioner Patton said there would be "hundreds more police … out and about" in coming days, with the city preparing to host big-ticket events like Christmas carols and the Boxing Day Test.

"We have no increased threat for any of those events … at this stage we believe and understand this is a quarantined incident," he said.

Emergency services were praised by the Premier for their quick response. ( ABC News: Gus Goswell )

The driver's vehicle was seen being removed from the scene overnight after police searched it. All roads in Melbourne's CBD were opened early this morning.

A second man, 24, who was seen filming the incident and found to be carrying a bag with knives in it, was also arrested yesterday afternoon.

He was interviewed by investigators last night but police now say he was not involved or connected with the initial incident.

He has now been released and is expected to be charged on summons over possession of cannabis and a weapon.

Witnesses described people crying and seeing blood after the crash. ( ABC News: Gus Goswell )

Premier Daniel Andrews said the crash had brought out "the very best in compassion and kindness" and praised the work of emergency services.

"This is a cowardly act, an evil act, one that has angered and I think saddened every single Victorian," he said.

"At the same time we've seen the action of a hero and we've also seen complete strangers reaching out to help each other."

Acting Lord Mayor Arron Wood said he was "sickened" by the crash but said people should feel safe to come into the city.

"Safety and security is always our number one priority," he said.