Metro's transit police arrested 20 Metrobus riders and handed out warnings to 623 others between Friday and yesterday mornings as part of a stepped-up anticrime campaign flowing from a one-day bus-driver strike last week.

Angus MacLean, chief of the transit police, said he diverted as much of his 161-man force as possible from the subway. His officers rode the buses in civilian clothes as part of the campaign.

All the arrests and warnings were issued in the District of Columbia and most of them were for such offenses as evading payment of the fare, or smoking, drinking and playing a radio on the bus.

Hundred of Metrobus drivers walked off the job last Thursday after a female driver was raped while on duty. The drivers complained that they have not received adequate police protection, particularly in comparison with that given to the subway.

Metro, city and transit union officials met all day Thursday and agreed on a six-point program that included improved enforcement on the buses. In addition, Metro promised to complete installation of a "silent alarm" system on all the buses. With that alarm, the bus driver can quietly signal Metro headquarters that he has an emergency. Police units are dispatched to the point where the bus is scheduled to be.

D.C. police will join the stepped-up on-bus campaign tonight, according to Assistant Chief Bernard D. Crooke. "We couldn't start anything on the weekend, because we had too many demonstrations in town," Crooke said. Officers dressed in casual clothes will be riding the buses regularly, he said.

Crooke said that a captain and a sergeant have been assigned full time to the Metrobus problem. "When you look at the raw statistics of crime," Crooke said, the Metrobuses "are not where we want to deploy the major bulk of our people . . . But I can sympathize more with the daily frustrations of the bus drivers in dealing with problems than a lot of people can. . . ."

MacLean said that nonpayment of fare and the smoking of marijuana on a bus automatically result in arrest. Other violations - such as smoking cigarettes, drinking and playing a radio - result only in warnings unless the bus rider become belligerent.

Of the 20 riders arrested, he said, seven have had their cases referred for a court date; seven were permit two were referred to the Juvenile Reted to post collateral of $25 or $10; ceiving Home and four were warned and released.

Metro technicians were nearing completion of their work on the silent alarm system yesterday and were testing it on some Metrobus routes.

George Davis, president of the Amalgamated Transit Union Local 689, which represents the drivers, said that "I feel from what I've seen that there definitely has been an all our effort by Metro to live up to its end of the bargain."