The incident happened in Cambridge's Arts Picture House, when the unidentified person who was sitting in an aisle seat took his penis out.
The manager quickly received a complaint about the man. Some 200 cinemagoers were present in the room at the time, according to SWNS.
Manager Keith Gehlert recalled: "He was just doing it in full view of everyone.
"I couldn't believe it - I told him he had to leave and waited until he got himself together."
Gehlert then called police after the man left. Authorities are currently appealing to the public for more information about him.
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