Two detectives shot a bystander in the leg yesterday while aiming for a pit bull in Brooklyn, cops said.

The undercover cops bought heroin from a dealer on Himrod Street in Bushwick at about 11 a.m. and pursued the suspect into a nearby house, cops said.

A pit bull lunged at the detectives, who fired two shots, sources said. One of the bullets hit a 22-year-old man fixing his bike outside.