D.C. police arrested 91 persons in a predawn gambling raid at a Northwest Washington residence yesterday and said they confiscated about $3,000 worth of drugs and large quantities of alcohol and gambling paraphernalia.

Police said the 5 a.m. raid ended a month-long investigation of an illegal after-hours gambling and drinking establishment at 1208 U St. NW. Two persons were charged with narcotics violations and seven others were charged with gambling violations, police said.

According to police, about 50 D.C. police detailed to the gambling unit converged on the residence where, it was said, an illegal gambling operation occupied the second floor with its own separate entrance. When a doorman let one of the participants out of the building, police rushed in, they said.

Police said that when they entered and announced that they had a warrant, some people began dropping drugs on the floor. Found, they said, were 16 foils of cocaine, 28 bags of heroin, a quantity of marijuana and a variety of mood-inducing pills. Police also confiscated large quantities of liquor from a well-stocked bar.

The raid came hours before D.C. Police Chief Burtell M. Jefferson issued an order that policemen assigned to the gambling and liquor squad will hereafter be assigned on a permanent basis. The order changes the temporary assignment status of those squads that was in effect since 1974 after several members of the police gambling squad and a dozen citizens were arrested in a gambling case.

After that 1973 gambling case, then-police chief Jerry Wilson ordered that police be assigned to gambling and liquor squads for only two-year terms to ensure a constant turnover of personnel to help prevent another such case.

His successor, Maurice J. Cullinane, kept the rotation system. However, police officers complained that they were rotated just about the time they became fully proficient.

Chief Jefferson ordered an investigation by the police department's planning division to determine the most efficient way of operating the squads.

On its recommendation, Jefferson issued the order that permanently assigns police to gambling and liquor squads if they pass investigators' examinations as members of other vice squads must do.