It happened on Graham Avenue in Bushwick as the 27-year old victim was walking early Friday morning.
The suspect, police say, came up behind her, threw her to the ground and then hit her head against the pavement and attempted to rape her.
That is when the bystander intervened, and scared the suspect off. Police describe the attacker as being in his early 30's, between 5'8" and 6'0" tall, and about 170 pounds.
The victim was taken to Woodhull Hospital and treated for her injuries.
Police are asking anyone with information to call Crime-Stoppers at 1-800-577-TIPS>
