The Alexandria City Council unanimously approved last night the idea of creating a city-run bus system as early as late 1983, when Metro subway service is scheduled to begin there.

"There's no question about this," said Vice Mayor James P. Moran (D). "We're doing it." The council called for precise determination of costs and other details before voting money for the project.

A city-operated system would replace some Metrobus service and feed commuters into Alexandria's three subway stops.

Alexandria is one of several jurisdictions to take steps to replace some lines of the area-wide transit system as a response to rising costs and to give local officials more control over service.

Earlier this month, the Fairfax County Board of Supervisors voted to begin operating its own public bus system and to withhold $6.3 million it owes Metro. The county plans to center its system on the Huntington Metro Station just south of Alexandria.

Alexandria Mayor Charles E. Beatley said last night that since the city already is paying $6 million to Metro for service from 135 intercity buses, "we're already in the bus business . . . . We're talking about transferring a part of that -- we're not doing anything vastly different. We're just going to be more in the driver's seat."

Beatley said a city-sponsored system will mean that Alexandria will pay considerably less for service that "we'll be able to tailor to our own needs."

A consultant has proposed that the city spend $2.4 million to buy 18 buses, according to Councilman Donald Casey (D). The consultant, who made studies for both the city and Fairfax, also said the city's saving in Metro payments would be similar to the $600,000 reduction Fairfax expects.

Council members said they believe time is of the essence if the city is to make the most of its new subway stops and avoid traffic congestion around them. "If we don't have the system in place when Metro opens," said Casey, "we're not going to look very good."

Montgomery County currently operates 74 buses in Silver Spring and 19 in Gaithersburg, with 52 more on order for Rockville and Bethesda. Prince George's County is planning to contract with private companies on some routes, and the District of Columbia has begun operating buses on a few routes.