A 91-year-old woman was choked and robbed at gunpoint by a man twice her size in Park Slope.

It happened inside an apartment building lobby on 9th Street around 1 p.m. Wednesday, police say.

Police say the suspect approached the woman from behind and followed her into her apartment building where he placed her in a chokehold and displayed a gun.

As the suspect began taking money from the woman, police say a 50-year-old passerby saw what was happening and tried to stop the man, but the suspect pointed the gun at him.

Police say the man got away with $370 then ran away.

He is described as being 40 to 50 years old, 5'10'' and weighing 250 pounds. He was last seen wearing a dark-colored hooded sweater, a black jacket, a black wool cap, black gloves, light-colored pants and dark-colored shoes. 

Police have released surveillance images of the suspect. 

Anyone with information is asked to call the NYPD's Crime Stoppers hotline at 1-800-577-TIPS.