A straphanger stepped on a man's foot on the subway and when confronted, punched and kicked the man in the face, police said.
The victim, a 30-year-old man, was on a southbound R train Friday afternoon when his foot was stepped on, the NYPD said.
When confronted the stepper, the suspect punched him in the face, police said. The force knocked the victim to the ground, where the suspect continued punching and kicking his face, police said.
The train stopped at the Fourth Avenue and 36th Street station in Sunset Park, Brooklyn, where the suspect got off and boarded a southbound D train, police said.
The victim was taken to Lutheran Hospital in critical but stable condition, police said.
The suspect is described as 17 to 19 years old; about 5 feet, 10 inches tall; and 160 pounds. He was last seen wearing a white T-shirt, black jeans, white sneakers and a black book bag.
Anyone with information is asked to call the NYPD's Crime Stoppers Hotline at 800-577-TIPS.