The number of rush-hour commuters driving into the downtown area has increased by nearly 6 percent this year, setting a record of 250,000 motorists, according to a new report by the Metropolitan Washington Council of Governments.

Many commuters have sought to avoid delays on heavily congested highways and bridges by starting out early or waiting until rush hour is nearly over, the report said. It cited sizable increases in traffic before 7 a.m. and after 9 a.m.

The survey, conducted during March, April and May, found an average of 168,050 cars heading into the District's central employment area, including downtown, Georgetown and Capitol Hill, between 6:30 a.m. and 9:30 a.m. on weekdays. The total represented a 5.9 percent increase over 1983.

According to the study, an average of 250,047 persons traveled into the central area by car, including 115,794 driving alone.

Another 132,537 rush-hour commuters traveled to the city's central area by Metro buses and trains, commuter rail and privately run bus lines, according to the report.

The number of mass-transit passengers marked a 7.4 percent increase over 1983, a gain attributed partly to the opening of Metro's Yellow Line extension to Alexandria and Fairfax County last December. The Metro system has previously reported similar increases in ridership.

Rush-hour traffic on local roads dropped sharply after the nationwide 1979 gasoline shortages, but began rising again in 1982 and 1983 as gasoline prices leveled. Metro ridership reached a peak in 1980 because of the gasoline crisis, but then declined for several years until the recent upturn.

With many roads and bridges already jammed at rush hour, transportation officials previously have predicted that more commuters will be forced to leave for work earlier or later than in the past. Some commuters also may switch to car pools and mass transit, officials said.

In the past two years, traffic into the District's central area between 6:30 a.m. and 7 a.m. climbed 10.8 percent, the report said, and between 9 a.m. and 9:30 a.m. it rose 9.8 percent. In contrast, traffic increased only 2.4 percent between 8 a.m. and 8:30 a.m. and 1.8 percent between 8:30 a.m. and 9 a.m.

In the past four years, traffic into the city across Potomac River bridges rose 41 percent both before 7 a.m. and after 9 a.m., the study said. But it increased less than 20 percent between 8:30 a.m. and 9 a.m.

Some Northern Virginia commuters also may drive to work earlier or later to avoid rush-hour car-pool requirements on I-66, city officials said.

The number of commuters driving alone has increased steadily since 1980, when the total was 92,097.

The report said commuters in car pools rose to 134,358 in 1982, then declined to 128,334 last year and this year returned to the 1982 level.

The council conducts two forms of surveys in alternate years. Next spring's survey will focus on a broader area, including the central portion of the District along with Rosslyn, Crystal City, the Pentagon and other parts of Northern Virginia.

In other findings, this year's report said the city's most heavily used truck routes were New York Avenue, South Capitol Street, the 14th Street bridge, Key Bridge and the Southeast Freeway.

The study also cited a sharp increase in van pools. Commuters riding in vans with seven or more passengers increased 112 percent in four years, from 5,810 riders in 1981 to 12,522 this year. The survey counted 473 van pools in 1981 compared with 980 this year.