One man lost his hand and another was critically injured when they were struck by a subway train early Saturday in Brooklyn, police said.

A Manhattan-bound M train hit the men at the Central Avenue station in Bushwick at around 5:15 a.m., said cops.

The train operator spotted one of the men standing on the tracks, his back to the train.

The operator braked, but the train still plowed over the men, authorities said.

The man who lost his hand, 57, also suffered head trauma, cops said. He was listed in critical condition at Bellevue Hospital.

The other man, 26, was treated for head injuries at Elmhurst Hospital, police said.

It was unclear why the men were on the tracks, but police are investigating alcohol as a factor, sources said.