Metro's "Downtowner" minibuses, which have provided convenient bargain-rate transportation for many years, are to carry their last foot-sore shoppers and government workers on Sept. 4.

The Metro board yesterday approved a request from the District of Columbia government to eliminate the service. About 2,400 people ride the buses daily on a route crossing commercial areas of downtown Northwest and Southwest.

The buses operate at six-minute intervals from 10 a.m. to 3:30 p.m. weekdays. Most passengers pay 25-cent fares rather than the basic 60 cents; senior citizens pay 10 cents.

The city hopes to save about $450,000 in subsides by eliminating the route, according to the city's mass transit director, Anthony Rachal. The service is duplicated by other bus routes and by the subway, Metro officials said.

"It's a bad move by Metro," said Tony Green, a 23-year-old government worker who was riding a Downtowner bus late yesterday afternoon. "It's very convenient to most of the downtown area. You don't need a car I take it every day because you can get to government buildings easily."

"I think it's a shame," Downtowner bus driver James Martin said yesterday when he heard of the board's decision. Martin said some of his riders were circulating petitions earlier in the week to try to save the service. "I see the majority of the same faces every day," he said. "There are a lot of government workers and also a lot of couriers. Rush hour for this bus is noon hour.I have trips with standing room only."

Downtown shuttle service originated under D.C. Transit, the private firm that until 1973 owned and operated the city's bus system. Subsidized by local merchants, light green shuttle buses ran up and down F Street, moving shoppers from block to block at a flat fare of a dime.

In recent years, increased fares and the opening of the subways bit into ridership, which fell from about 3,700 passengers per day in February 1976 to 2,400 last February.

Yesterday the Metro board also voted to eliminate the H-6 route next January. That route links the Brookland Metro station with the Kennedy Center, and is used primarily as a connector, Metro officials say. A new line, H-1, will be created to link Brookland and the State Department, but will operate during rush hours only.

Metro's plan also would alter the routes of the M-F, D-5 and D-6 buses in Northwest and Maryland. The M-4 would take over part of the D-5 and D-6 routes west of Wisconsin Avenue, with service eliminated to the community of Brookmont, located just across the D.C. line in Maryland. Brookmont generates an average of only two passengers per trip, according to Metro.

During rush hours the D-6 will link the 31st Street-Western Avenue intersection ot Tenley Circle, but during off-peak hours service will extend only as far as Friendship Heights.

In other action yesterday:

The board voted to make permanent two experimental lines inaugurated in January. The two are the U-6, which links the Minnesota Avenue Metro station and Marshall Heights, and the W-6, which runs between the Potomac Avenue station and Fairfax Village.

The board awarded a $9 million contract to the West German firm M.A.N. for the pruchase of 30 articulated, or jointed, buses. The 60-foot vehicles are to begin arriving in July 1983 and are expected to be used in the District of Columbia. The U.S. Transportation Department yesterday announced a grant of $3.8 million to Metro to help in the purchase cost.

The board voted to allow advertising on the sides of Metrobuses, a step intended to increase revenues. Ads in the past were limited to the inside and rear exterior of the buses.

The board gave final approval to allow motorists to park free on Saturdays at nine parking lots that Metro operates at rail stations.