The Metro Transit Authority took a legal step yesterday to block Fairfax City - at least temporarily - from replacing Metrobus service with chartered commuter bus service into downtown Washington.

In an unusual action, endorsed by the Metro board of directors, Metro lawyers asked the Washington Metropolitan Area Transit Commission not to approve at this time the rates that Gray Lines, Inc., plans to charge for hiring the buses to Fairfax City and running them. Current plans call for service to start Jan. 2, using seven buses.

The proposed tariff sets special rates Gray Line would charge Fairfax City for running the new service on a regularly scheduled basis. The proposed rates are lower than those Gray Line would charge under existing tariffs, which cover occasional users of chartered buses.

Without such a tariff, the city would have to pay the higher rates or, depending upon complex legal circumstances, might not be able to run the service at all.

Metro's action is the latest volley in a running feud between the transit authority and the small suburban city. It began when Fairfax City sued Metro, claiming it had been cut out of subway construction plans, and won a $2 million judgment. Metro is appealing the decision.

The Fairfax City Council voted unanimously Nov. 8 to pull out of the Metrobus system, chiefly to save the money that the city must pay to subsidize Metro operations. The next year has been estimated at $280,000.

Under the new charter bus plan, the city has agreed to pay Gray Line the difference between what is collected from ticket buyers and the cost of chartering the buses.

The city already has started selling 10-ride tickets for the new service. For city residents, the tickets cost $11, or $1.10 a ride. For nonresidents, the tickets cost $13 or $1.30 a ride, the same as on a Metrobus.

In its petition yesterday to the transit commission, Metro said its board plans to hold a public hearing Jan. 4 on possible curtailment of service to Fairfax City. The hearing will start at 7:30 p.m. at the city hall.

Metro asked the transt commission to suspend - in other words, not to approve - the tariff until at least Feb. 1, allowing Metro time to reach a decision on what service, if any, to eliminate.

William H. McGilvery, executive director of the transit commission, said that body's three members were promptly informed of the Metro request, and a decision will be reached soon.

The transit commission was created to regulate fares and service at a time when all public transit service in the region was operated by privately-owned companies. When Metro took over those companies in 1973, the commission continued to oversee such private carriers as Gray Line, chiefly a sightseeing company.

In its petition to the commission, Metro noted that the commission not only has power over fares, but also the power to restrain competition by private companies against the publicly owned Metro system.

Theodore C. Lutz, Metro general manager, said the Fairfax City bus service could set a troublesome precedent that could lead to other competitive transit services. He sought and got the Metro board's approval for filing yesterday's petition with the commission.

Fairfax City Mayor Nathaniel F. Young, contacted later, voiced annoyance with Metro's action. He said he was surprised to learn a legal question could be raised against the Fairfax City charter plan.

"But I'm not going to ulcerate over this," Young said. "I expect the city attorney to work with Gray Line to get this cleared up, but the important thing is that there will be bus service for commuters on Jan. 2, any way it goes."

On another matter, Metro board chairman Francis W. White announced an agreement with the U.S. Department of Transportation assuring that Metro can pay $26.2 million interest on subway construction bonds due Jan. 1. Of that money, $22.9 million will come from the federal government.

The issue got hung up last week because the Transportation Department's Urban Mass Transportation Administration (UMTA) was insisting upon various conditions before releasing the U.S. funds.

One condition would have bound Metro to a deadline of next June 30 to complete a full financial plan - a deadline Metro officials regarded as impossible to meet, since the ultimate length of the rail system that must be financed is not yet known.

White, Lutz and others met with Richard Page, UMTA, administrator, and won his agreement to extend the financial plan deadline to next Aug. 30. Other conditions also were eased.

Told of the agreement, the Metro board unanimously approved a resolution to make the interest payment.

The board also approved various bus service cutbacks in Alexandria, starting next February and March, that will save an estimated $240,000 in operating costs.