A B-double tanker driver has been arrested after leaving the scene of a crash in downtown Melbourne that left five pedestrians injured, two of them critically.

Witnesses told police the truck left the road and hit a traffic light near the intersection of City Road and Power Street in Southbank about 7pm.

"It appears that while performing the left-hand turn the truck cut the intersection corner and mounted the footpath, taking out the traffic light which fell to the ground," Victoria Police said, in a statement.

Police had earlier said investigators were told the truck had collided with the group of people as well as the traffic light.

Four men and a woman, all believed to be aged in their 20s and 30s, were injured.

Police said the vehicle left the scene via Power Street.

A driver who saw the crash followed the tanker and passed on information that led to police finding the truck at Truganina, about 20 kilometres west of Southbank.

Police arrested a 64-year-old driver from Wyndham Vale, who detectives were speaking to overnight.

Major collision investigation unit detectives appealed for anyone who saw the incident or had dashcam footage to come forward.

Ambulance Victoria said two of the injured men, believed to be in their 20s, were taken to the Alfred Hospital. One was in a critical condition, the other was serious but stable.

The other pedestrians, one critically injured and the other two in a serious but stable condition, were taken to the Royal Melbourne Hospital.

Paramedics said all five suffered lower-body injuries.

Harry Ledingham, who works at My Goodness nearby, said he didn't realise what had happened until he heard the sirens.

He said an "army of fire trucks" was blocking the scene for about an hour and a half and an investigation team remained afterwards.

"It's shocking," he said.

"It's a bit of a reality check. That sort of stuff doesn't happen every day."

The crash scene is expected to remain cordoned off throughout the night.

Anyone with information should call Crime Stoppers on 1800 333 000.