Police spokesman Anthony Guglielmi told The Baltimore Sun Detective Brian Stevenson, 38, was struck on the head with a brick or piece of concrete Saturday night.
Stevenson, an 18-year police veteran, was out celebrating his birthday, CNN reported.
Guglielmi said Stevenson got into a dispute with a group of people outside a restaurant over a parking spot.
At a news conference early Sunday, Police Commissioner Frederick Bealefeld said Stevenson was pronounced dead at Johns Hopkins hospital, CNN said. The chief said witnesses have been cooperative and a suspect was in custody but hadn't yet been charged.