The shooting happened about 12:50 a.m. at the Century Theaters, 32100 Union Landing Blvd.
Police said there was a disturbance between several patrons in the theater before the shooting.
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"During the dispute, the suspect fired a handgun. There were no additional reported injuries or victims," police said in a news release.
The suspect left before police arrived. Officers evacuated the theater complex and found one man with an injury. He was hospitalized with non-life-threatening injuries.
Union City Police Detectives are asking anyone who may have been at the theaters or who witnessed anything related to the case to contact Detective Smith at (510) 675-5268 or the Union City anonymous tip line at 510-675-5207.