UPDATE: A THREE-month-old baby and a two-year-old are among those injured in today's tragedy in Melbourne.

A spokesman for the Royal Children's Hospital in Melbourne said the three-month-old baby was currently in surgery and the two-year-old was in intensive care.

The hospital is also treating a nine-year-old and a twelve-year-old. Staff also treated a 23-year-old with limb injuries.

The spokesman said the nature of the injuries ranged due to the nature of the trauma.

In a press conference, police told media the three dead including a child, a man and a woman at different locations along Bourke St.

Victoria Police Commissioner Graham Ashton said the suspect was a 26-year-old man who had been arrested less than a week ago on domestic violence allegations.

He is alleged to have assaulted two members of his extended family.

Commissioner Ashton said the man had an extensive history of family violence and drug issues.

Police earlier said the man had allegedly taken a woman hostage in the same car he used to deliberately mow down pedestrians.

The woman, who was known to him, managed to escape the car.

The incident will be the subject of a coronial enquiry.

UPDATE: ONE of three people killed in today's incident was a young child.

Victorian Police confirmed one of the victims was a child.

Many of the injured are in care at the city's hospitals, including four patients admitted to the Royal Children's Hospital.

In a statement on social media, the hospital said one patient was in a critical condition and the other three patients were in serious condition.

Ten patients are being treated at the Royal Melbourne Hospital.

Police have also confirmed officers shot the man accused of deliberately crashing into pedestrians.

"Police shot the man who was the sole occupant of the vehicle and he received bullet wounds," a statement said.

"He was taken into custody where he is being treated for non-life threatening injuries."

Police said the man was also accused of taking a woman known to him hostage earlier in the day.

"She managed to escape from the car on the Bolte Bridge earlier today," the statement said.

UPDATE: VICTORIA Police have assured the country today's horrific incident was not terrorism.

Victorian Premier Daniel Andrews is due to join emergency services to brief the media at 5pm Melbourne time.

Three people have been confirmed dead and the driver is in custody.

This guy tries to stop the driver with a baseball bat #BourkeSt pic.twitter.com/2fnQfyznuC — Paul Osborne AAP (@osbornep) January 20, 2017

UPDATE: VICTORIA Police say three people have been killed after a man deliberately drove into a crowd of pedestrians in the Melbourne CBD.

A police spokesman said police believed the incident was connected to an earlier stabbing.

He said details were still emerging.

Police are due to brief media again at 5pm.

Melbourne pedestrian deaths not terrorism: police: Acting Commander Stuart Bateson briefs police on the drama in Melbourne after three people were killed and 20 injured when a man in a Commodore ploughed into CBD crowds. Melbourne pedestrian deaths not terrorism: police: Acting Commander Stuart Bateson briefs police on the drama in Melbourne after three people were killed and 20 injured when a man in a Commodore ploughed into CBD crowds.

UPDATE: POLICE reportedly shot a man and then arrested him at the scene of a horrific crash in Melbourne where a man drove his vehicle into a crowd of pedestrians.

The Herald Sun reported police shot a man before arresting him.

UPDATE: ONE person is dead and at least 20 injured in Melbourne's CBD amid reports of shots fired and a police car chase in Bourke St.

At least 20 people are being treated by paramedics after the incident at Bourke Street Mall where bystanders were hit by a car being driven erratically.

Police said a "number of people were injured" in the CBD. One person has been confirmed dead.

The driver had been arrested and the situation was now "contained", Victoria Police said.

An ambulance spokeswoman told news.com.au they were aware of at least one person being struck by a car. The spokeswoman could not say how many ambulances were heading to the scene.

A witness said the driver of the car being chased "hanging out the window screaming" and described as a thin, dark hair man who appeared Middle Eastern.

There are reports from eyewitnesses a pram has been struck, with images of an upturned pram flooding Twitter.

The dramatic scene at Bourke St Mall. Picture: TWITTER/@davidhurleyHSSource:Twitter

A witness told Fairfax she saw a baby fall under a car. Another witness said at least three people had been hit by the car.

"There was a car that just sped along the footpath and hit people," a witness told police at the scene.

A witness, Chris, told Sky News: "There were two people [taken in an ambulance]. Another rushed into a police car. I understand there are more people down the road (near King St). Just a hell of a lot of blood.

"I feel pretty moved and quite sick. It's a horrible thing to have had happened. I was standing by the pram for a while and when police got here they screamed for everybody to get back and move."

Two American tourists, Michelle and Daryl, said they saw the pram lying on the side of the street. People were running everywhere as were police with their "guns drawn".

"I heard at least five shots. Then we started trying to find cover in one of the buildings," Daryl said.

Michelle added: "(I was) stunned when I saw them drawing their weapons. People started screaming. We ran the opposite direction of police."

The police operation on Bourke Street in the Melbourne CBD. Picture: Twitter.Source:Twitter

Police received a flood of calls about 1.30pm.

Another witness said: "There was a whole lot of crashes and bangs and there was a mess of bodies left behind."

The tram system in the city has been suspended.

One witness said some of those at the scene were in tears at what had unfolded, The Herald Sun reported.

He said police were stopping people leaving the RACV Club.

Emergency services attend to a man after the incident. Picture: TWITTER/@PeterK0243Source:Twitter

That man said a car was trying to escape police when the incident happened.

"I heard a thump and thought they'd hit something. Now I know it was people," he said.

Police from across the city converged on the scene within minutes.

UPDATE: VICTORIA Police have confirmed at least one person has died after someone drove a vehicle into a crowd of shoppers in the Melbourne CBD.

In a statement, police said the exact circumstances had yet to be determined.

One man has been arrested, police said.

Paramedics have treated up to 20 people.

EARLIER: POLICE are chasing a rogue car which has driven erratically through the Melbourne CBD.

Ambulance Victoria confirmed paramedics had reports multiple people had been hit by a car.

A number of unmarked Police cars and a police helicopter have been deployed in the chase.

Paramedics are still unsure how many pedestrians had been hit.

Emergency services were flooded with calls about 1.30pm.

Breaking #melbourne, a car rushed in the crowd of shoppers in #bourkestreet, hitting pedestrian. Police is circling the area pic.twitter.com/128aZe1hMC — Mp Desset (@_mp_d) January 20, 2017

People ran for their lives as the hoon sped through busy and oncoming traffic, narrowly missing pedestrians.

The hoon sped off down Swanson street before Police could arrive on the scene.

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A police helicopter is hovering above the city.

Witnesses shared their experience on social media, saying the vehicle crashed into a crowd of shoppers.