An unstable man set off a firework in his ex’s Brooklyn apartment building and smashed parked cars with a hammer Friday morning, police sources said.

The incident unfolded inside 446 Madison St. in Bedford-Stuyvesant just before 8 a.m., according to police.

Sources told The Post the man, whose name was not released, lit the fuse to a firework across the hall from his ex-girlfriend’s apartment, damaging a neighbor’s door.

He then ran outside and used a hammer to smash the windows of cars before taking off on a dirtbike, according to sources.

The suspect’s ex-girlfriend told cops the man was suffering from a psychotic episode, sources said.

Cops are still searching for the man.