Terrified witnesses have described the moment pedestrians were mowed down by a vehicle that ploughed through Melbourne's CBDleaving 19 people hurt.

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.

Witnesses estimate he was driving at about 100km/h and ran a red light. The alleged driver, a 32 year old man, and another man, who may not be related to the incident, have since been arrested.

One witness said the alleged driver “hit the first lot of people” then revved the engine again.

“It was quite deliberate,” he said.

Victoria Police have identified the alleged 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.

One witness said the crash "sounded like an explosion".

"I sprinted down seconds after the crash and there were just bodies everywhere," he said.

Rohit Kaul, 45, was on the phone to his wife Avi as he crossed Elizabeth Street before he was hit by the car, Fairfax Media reports.

He suffered leg injuries and was helped by pedestrians before being transferred to the Alfred Hospital.

Sue, another witness, told 3AW “people were flying everywhere”. A witness named Frank said he counted “five cop cars and an ambo coming up the tram tracks”.

“Beat coppers were running down on foot too,” he said.

Lachlan Read, 20, told News Corp the car went through a red light and that the whole incident lasted about 15 seconds.

• 'Deliberate act': Horror in Melbourne CBD as vehicle ploughs into pedestrians

• Pictured: The two men arrested after car hit pedestrians in Melbourne

“He has gone straight through the red light at pace and it was bang, bang, bang,” he said.

“It was just one after the other. The last bang the car stopped. There were bodies on the ground and people running up to them — it was mayhem.

“It is definitely something you don’t expect to see on Flinders St at 4.45pm.

Paramedics are assessing 19 people at the scene and four people have been taken to hospital in a serious condition, according to Ambulance Victoria.

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.

A four-year-old boy with a head injury has been taken to the Royal Children's Hospital in a stable condition.

The extent of other injuries are unknown at this stage.

The incident has brought parts of the city to a standstill four days out from Christmas.

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.

It comes almost a year after a car ploughed through pedestrians on Bourke Street.