A young man is in a critical condition in hospital after falling from a moving taxi in Sydney's inner west.

Police say the 18-year-old suffered severe head injuries when he fell onto the Princes Highway at Tempe.

It is unclear how he fell because officers have been told the man was sitting in the back seat of the cab.

His friend, who was also in the taxi, says the driver forced them out because they changed the way they were headed after they got into the taxi at Newtown.

"We were going from King St to Miranda," the man said.

"Then I said to him we don't want to go to Miranda anymore we want to go to Newtown so then we were going to turn and then we got pushed out of a moving vehicle."