UPDATED 9.00PM A 10-year-old child is among three people who've been killed in Melbourne today by a man with drug, violence and mental health issues who deliberately mowed pedestrians down with a car in the Bourke St mall.

A man and a woman in their 30s were also killed by the 26-year-old driver, who's now in hospital under police guard following his dramatic arrest after the car crashed in the CBD.

Police commissioner Graham Ashton says 15 more people were injured by the car in the attack, five of whom are critical.

LISTEN ABOVE: NZME'S MIRIYANA ALEXANDER SPEAKS WITH CHRIS LYNCH FROM THE SCENE

The Royal Childrens' Hospital said four children, aged three months, two, nine and 12, are in a critical condition.

Amid the carnage on Bourke St, half a mangled pram was stuck on the bonnet of the car with what appeared to be the other half on the ground at a pedestrian crossing around 100 metres further up the road.

Victorian police chief commissioner Christine Nixon was at the scene and said she saw a pram on the ground and helped the mother.

"She kept asking me 'where's the baby?'," Ms Nixon told ABC 774.

Our prayers and deepest sympathy are with the victims & their families, of this vicious criminal attack in Bourke St Melbourne today.

— Malcolm Turnbull (@TurnbullMalcolm) January 20, 2017

Police believe the man had been on a criminal rampage since the early hours of Friday.

He is accused of stabbing his brother in Windsor at around 2am, and at around 11.45am police tried to pull him over on the Bolte Bridge armed with guns and tasers but aborted their pursuit because his driving was too dangerous.

Police say a woman, who may have been hostage in the man's car, jumped out of the moving vehicle.

The man was next spotted doing burnouts outside Flinders Street Station before moving down Swanston St and then mowing people down in Bourke St mall.

Police everywhere. Scary scenes in #bourkestreet #melbourne pic.twitter.com/C27BmxOvw1

— Jess Minear (@jessy_louu) January 20, 2017

Mr Ashton said he could not confirm whether two cars seen following the driver on Swanston St were police cars.

The man was also injured, shot in the arm by police who fired at least five times at him before his car crashed and he was arrested.

He was seen lying on the ground dressed only in red underwear and white shoes with several police offices standing over him.

Feeling thankful to have missed this by minutes. One minute shopping, the next madness descends #Melbourne pic.twitter.com/kSxsqvXtRX

— Miriyana Alexander (@miriyananz) January 20, 2017

The three dead were each found at separate locations along Bourke Street, Mr Ashton said, suggesting they weren't related.

Witnesses reported hearing at least five gunshots before the man was arrested after ploughing into people in the mall.

"There's someone on the middle of the ground and there's blood all over the street," Rishi Hathiramani, a CBD shopkeeper, told AAP.

A number of CBD businesses went into lockdown, while others had been evacuated.

City office worker William Mulholland said he came out of a laneway and saw a number of bodies lying on the pavement receiving CPR from police and emergency workers.

He said his friend came out of the RACV building on Bourke St and saw a body lying on the pavement in front of him.

Victims have also been taken to the Royal Melbourne Hospital and Alfred Hospital.

A number of people have also gone to hospitals in shock after witnessing the incident, police said.

BREAKING: 3 people killed & several others injured after man deliberately drove into pedestrians in Melbourne, Aus.pic.twitter.com/NlJSh5SzjS

— Breaking911 (@Breaking911) January 20, 2017