Nineteen people including a four-year-old boy are in hospital after a car ploughed into pedestrians in central Melbourne.
Horrified witnesses have described bodies "flying everywhere" when a white 4WD drove into a crowd on Flinders street between Elizabeth and Swanston streets shortly after 4.30pm.
It comes almost a year after a car ploughed through pedestrians on Bourke Street.
Emergency services were quick to arrive at the scene and police have arrested the driver of the car, a statement from Victoria Police media said.
Victoria Police have identified the driver as a 32-year-old Australian citizen of Afghan descent.
Another male, aged 24, who may not be related to the incident, was also arrested at the scene, police said.
Victoria Police North West Metro Region Commander Russell Barrett said police believe the crash was deliberate.
“We believe based on what we have seen that it is a deliberate act. The motivations are unknown. It is still the early days of the investigation," Commander Barrett said.
When asked if the crash was terrorism related he said it was too early for police to comment on that.
Commander Barrett said police will continue to have a strong presence in the area overnight.
“People may have items and vehicles within the crime scene. At this stage, they will not be able to be retrieved until the crime scene is reopened," Commander Barrett said.
“We understand that this has been a very traumatic event, and there will be witnesses, victims and family members affected.
"Anyone who witnessed the incident is urged to attend the Spencer Street police station at 313 Spencer St, Melbourne, to make a statement. Welfare services are present at that location."
Paramedics treated 19 people at the scene before taking them to various hospitals across the city, with four in a critical condition, according to Ambulance Victoria.
A pre-school aged child with a head injury has been taken to the Royal Children's Hospital in a stable condition.
Another person has been taken to the Alfred Hospital. Their condition is unknown.
The victims range in age from 25 to 83 years.
The extent of other injuries are unknown at this stage.
Witnesses at the scene say the driver of the car failed to stop at the red light as pedestrians crossed the road, with one bystander estimating the 4WD was travelling at 100kmh.
"As it (the car) approached this intersection ... it just mowed everybody down, people were flying everywhere," witness Sue told 3AW.
Another witness said his wife was waiting at the lights before noticing the vehicle coming up the tram tracks behind her.
He says after it hit the first group of people she heard the engine rev again.
Aerial footage of the scene shows a white SUV crashed into a tram stop.
The incident has brought parts of the city to a standstill four days out from Christmas.
Victorian Premier Daniel Andrews thanked the efforts of the emergency services on Twitter following the "horrible incident".
"As you will know, there's been a horrible incident on Flinders St.," he wrote.
"Stay safe. Check on your loved ones. And thank you to our brave emergency services."
Prime Miniser Malcolm Turnbull echoed Andrews' sentiments, offering his thoughts to the victims and rescue teams.
"As our federal & state police & security agencies work together to secure the scene and investigate this shocking incident our thoughts & prayers are with the victims & the emergency & health workers who are treating them," he wrote on Twitter.