Three pedestrians have been killed and at least 15 injured after a car was deliberately driven into a central Melbourne mall today.

A 26-year-old man has been arrested following the incident in the crowded Bourke Street Mall which police said was not terrorism related and believed to be related to an earlier stabbing in the suburb of Windsor.

Victoria Police Acting Commander Stuart Bateson told reporters he could confirm "that a male deliberately drove into pedestrians in the Bourke Street Mall."

"We have the offender in custody," he added. "There is no further threat to the public at this stage."

The three killed included a man and a woman aged in their 30s and a child, none of whom are believed to be related. Of the injured, police said five were in a critical condition including an infant.

Victoria Police Chief Commissioner Graham Ashton said the vehicle entered the city about 1.45pm and was driving erratically, doing "burnouts" on the corner of Flinders and Swanston Streets, before heading down Swanston Street and then turning left into the Bourke Street Mall where it struck pedestrians.

"We believe, at this point in time...that was intentionally..." Chief Commissioner Ashton said. The car then continued along Bourke Street before police rammed the vehicle outside the RACV Club and a man was taken into custody. The man was shot by police in the arm during the arrest and is being treated in hospital for what are believed to be non-life threatening injuries.

Victorian Premier Daniel Andrews described the incident as a "terrible, criminal act" and paid tribute to emergency workers and those "ordinary Victorians" who "stepped forward" to help others in the wake of the incident.

"Our hearts are breaking this afternoon..." he said. "But we are stronger than this...evil, criminal act today."

Police have asked people to steer clear of the CBD tonight where there are major disruptions to public transport services.

Anyone concerned about locating a loved one can call 1800 727 077. Witnesses and people with information can call Crime Stoppers on 1800 333 000.