A fast-moving Brooklyn fire displaced 29 people and left one firefighter injured, authorities said.

The three-alarm blaze broke out shortly before 11:40 p.m. Friday in a two-story home at 1948 85th Street in Bensonhurst, according to fire officials.

The FDNY tweeted Saturday night that the cause of the blaze was “accidental, careless discard of smoking materials.”

“Everything happened so fast. We didn’t even realize their was a fire,” said shaken first-floor resident Mira Sariyeva, who managed to grab her jacket and soap.

The fire started on the second floor, but flames and smoke quickly spread throughout the building and the building next door, the FDNY said.

The FDNY had the fire under control shortly before 3 a.m., according to a department tweet.

The Red Cross reported helping seven families and those left out in the cold by the blaze.