Justin Bieber's trail of liquor and litter at his Beverly Hills condo has set his neighbors off ... and now we know the cops have talked to them about placing the singer under citizen's arrest.

TMZ broke the story ... cops came to the condo 6 TIMES over the weekend -- 4 times for excessive noise in the middle of the night. Bieber partied hard -- 75 guests on Saturday night and 20 on Sunday.

The pictures on the rooftop deck tell only part of the story. People in the building tell TMZ ... the hallways and elevators reeked of pot. And several guests say drug use was obvious.

One person says, "There were bimbos lining up to do drugs in the lobby bathroom."

Law enforcement sources tell us ... cops have spoken to residents, Bieber and his management to resolve the situation.

We're told police informed residents cops can't make an arrest for a misdemeanor if they don't see it happen ... but the residents can take matters into their own hands by placing Bieber under arrest.

Via TMZ