A driver in Sydney's west was slashed in the face by a stranger after he stopped in the middle of the road, sparking a search for the alleged attacker.

The 28-year-old man was driving a white sedan down King Georges Road near Beverly Hills and Wiley Park at about 9.50pm on Thursday when the incident occurred.

Police allege he was stopped by a man who was unknown to him.

When he stepped out of his car, police allege the driver was slashed by the stranger, who fled the scene. 

The man then drove about 1km to The Street Kid Cafe at Punchbowl. 

Witnesses told 7News they called emergency services after finding the driver with severe cuts to his face. 

The man was treated at the scene before being taken to hospital in a serious but stable condition.

Police have launched a search for the alleged attacker. Anyone with information or dashcam footage is urged to contact Crime Stoppers.