Man critical after hit-and-run

The driver of a silver sedan is being urged to hand themselves in after a hit and run accident in Sydney's west has left a man fighting for his life.

A 51-year-old man was crossing Parker Street in Penrith, near the intersection of the Great Western Highway, at 7.30pm on Tuesday when he was hit by a car.

A 22-year-old witness says he pulled over after seeing a lifeless body lying in a pool of blood next to the road.

He and his passenger stopped their car and redirected traffic until emergency services arrived on scene, he told Macquarie Radio on Wednesday.

Police are searching for a silver sedan, similar to a Honda Civic.

Acting Inspector Peter Murphy said the driver should turn themselves into police.

"The car did slow after the impact so it would indicate (the driver) would have some awareness to what has happened," he told Macquarie Radio.

The victim is in Nepean Hospital with critical head and internal injuries.

Anyone with information should call Crime Stoppers on 1800 333 000 or go to www.police.nsw.gov.au/.