Inbound lanes on Melbourne's Citylink have been reopened after a crash this morning which left a large truck dangling from an overpass.

Crews are still working to clean up debris, but two inbound lanes have been reopened at a reduced speed.

All outbound lanes are open, but there are long delays stretching from Kings Way to the Bolte Bridge.

Citylink says damage to the overpass is being assessed and repair work will start immediately.

Crews spent much of the day working to retrieve the truck, which collided with a car and bounced across the freeway, smashing into a side barrier on CityLink in early peak-hour traffic.

Officials have moved the truck from its precarious position, and inbound lanes have reopened between Flemington Road and Footscray Road.

Police say the large truck was travelling southbound in the right-hand lane on CityLink at around 6:30am (AEST) when a car tried to merge in front of it.

It is believed the truck jackknifed while trying to take evasive action and then bounced across the freeway coming to rest on top of one of the side barriers.

The trailers dangled precariously over the side of the bridge and the driver fell from the upside down cabin.

He landed on top of some concrete debris 15 metres below and is in a critical condition at the Royal Melbourne Hospital.

The driver of the car that hit the truck has been taken to the Alfred Hospital with minor injuries.

The injured truck driver is carried to a waiting ambulance ( ABC TV )

The accident caused rush-hour traffic chaos across the city this morning, with one eyewitness describing it as being like a scene out of a Hollywood movie.

Seven hours after the crash, a crane carefully lifted the truck off the side of the bridge and on to the roadway, as trains on the Upfield line were temporarily halted as a precaution.

The MFB brought in a small drone to take pictures of the side of the truck that they could not see from the bridge and a cherry picker was used to examine the vehicle before it was moved.