As part of a cost-cutting effort the Metro transit system is scheduled, starting today, to halt most bus service across the 14th Street bridges between Northern Virginia and the District of Columbia and to reduce service on many bus lines in the District.

The long-planned shifts in bus service have prompted complaints from Northern Virginia commuters, many of whom will face inconvenience and delays. According to Metro officials, 7,900 bus riders will have to transfer at the Pentagon either to subway trains or to special shuttle buses. Because of the transfers, their trips may take five to 15 minutes longer, officials said.

By curtailing most service over the 14th Street bridges, Metro officials seek to avoid a costly overlap between the transit authority's bus and subway systems. Metro's Yellow Line, which opened April 30, crosses the Potomac River on a bridge parallel to the 14th Street span.

The cutback in bus service is expected to result in a substantial increase in passengers on Yellow Line trains. According to Metro's estimates, the decrease in bus service across the 14th Street bridges will save the financially strapped transit system $1.7 million a year.

In addition, the reductions in bus service in the District are expected to yield $1 million a year in savings. Metro officials said the new schedules require longer intervals between buses on lightly used lines. They said that relatively few D.C. passengers will be inconvenienced.

Other changes in bus routes and timetables also are scheduled to go into effect today, including some in suburban Maryland. These stem from recent shifts in traffic patterns and requests for route revisions from neighborhoods served by bus lines. Several timetables have been updated to reflect faster bus rides that have resulted from last December's opening of the final 10 miles of I-66 in Northern Virginia.

The Northern Virginia routes on which bus service will end at the Pentagon, include the 6, 7, 8, 9, 12, 14, 15, 16, 17, 18, 19, 21, 27, 28G and 29.

A new shuttle bus line, called Route 13, will run between the Pentagon and downtown, traveling across both the 14th Street and Memorial bridges.

Schedules have been revised for Northern Virginia buses running on Routes 1, 5 and 11.

In the District, routes on which service has been reduced include the B6, D, E, G, L, M, N, P, S, 30s, 40, 50s, 60, 70, 80s and 90s. Most changes entail increases of several minutes between buses. Service cuts on some lines are scheduled only on weekends.

Service on A routes in the District will also be modified.

In Maryland, timetable changes affect the D12, J, L, S12, T18, W and Z routes.

Information about revised timetables and routes may be obtained by telephoning Metro at 637-2437.