A man has been run over and seriously injured by a taxi in Sydney's inner west.

About 2.40am (AEDT) on Saturday, the 28-year-old was driving his taxi along Illawarra Road, Marrickville at the completion of his shift.

As he approached a pedestrian crossing the driver slowed to allow a group of men to cross.

Police investigators have been told that during a discussion the men sought to hire the cab but were told the driver had finished his shift.

As the driver moved away from the crossing it appears a 25-year-old man went under the front of the taxi and was run over.

The injured man was treated at the scene and taken to Royal Prince Alfred Hospital in a critical condition.

The driver was also taken to hospital to undergo mandatory blood and urine testing.

Police are appealing for any witnesses to come forward.