The number of passengers using the Washington area's mass transit system has climbed to the highest level in 30 years, according to Metro officials.

In April, officials said, ridership on Metro buses and subway trains rose to an average of more than 690,000 trips a day, for the first time surpassing a mark set during the nationwide gasoline crisis in 1979 and 1980.

Metro officials said that bus and rail patronage appears to have reached a level that was last recorded at the end of the Korean War, when the Washington area was served by privately owned bus and streetcar companies.

The gains have been attributed to a range of factors, including expansion of the Metro subway system, the absence of a fare increase this year, reduced unemployment and improvements in the reliability of transit service.

"This will be our highest year, no doubt about it," said John Fularz, an economist for Metro. "We're attracting a lot of new riders."

Robert A. Pickett, Metro's assistant planning director, said that ridership appears likely to rise further in coming years because of continuing growth in employment and increasing traffic congestion on roads leading to the central business sections of the District and Northern Virginia.

In addition to the increases in Metro's ridership, local bus systems have expanded. Ridership on Montgomery County's Ride-On bus system rose to an average of 26,000 trips a day in April, an increase of 7,000 trips in one year. Alexandria's year-old DASH bus system already is carrying almost 4,000 riders a day.

After reaching a peak during the Korean War period, mass transit patronage declined markedly over the next 20 years in the Washington area and in other major cities because of increases in auto ownership and deterioration in privately managed bus service.

With the public takeover by Metro of the area's private bus companies in 1973 and the opening of the subway system three years later, ridership began to increase. In 1980, Metro ridership set a record attributed largely to gasoline shortages that caused many commuters to switch from cars to mass transit.

During the next three years, Metro's ridership dropped because of readily available and cheaper gasoline, high unemployment, fare increases and more frequent breakdowns by subway trains and buses. The trend was reversed in mid-1983 and the upturn has continued.

Last month's ridership total -- an average of 693,411 bus and subway trips a day -- exceeded Metro's previous record of 674,383 in June 1980. Moreover, ridership for the current fiscal year, which ends June 30, is expected to average 656,000 trips a day, about 34,000 more than in fiscal 1980.

Total ridership for the current fiscal year has been projected at 192 million trips, the highest level since 1954, when the private bus and streetcar companies recorded 196 million rides. Metro officials said current ridership actually may exceed the 1954 level because the earlier statistics were not adjusted to reflect transfers between buses and streetcars.