The incident happened early Sunday morning when police were called to break up fights and control a crowd outside the Select Lounge, The Baltimore Sun reported.
"There was an altercation that took place very near the club and some officers worked to intercede in that fight, at which time some gunshots were discharged," said Police Commissioner Frederick H. Bealefeld said. "Several officers fired multiple shots."
Investigators were piecing together a time line of what happened, but an 8-year veteran of the force was killed. The officers' names were not disclosed.
"We're a few hours into this whole ordeal, and we have scores of detectives working on the case, processing evidence, interviewing witnesses," Bealefeld said. "We have a ton of work to do to put together the facts of what happened."
Dozens of witnesses were taken to the homicide unit for interviews, the report said.
"What we need to figure out is what sparked the shooting," said Anthony Guglielmi, a police spokesman. "Was there a weapon drawn by a civilian? Was the officer's weapon taken? We've got to put together a timeline and figure out what happened."