District officials reduced the speed limit on the South Capitol Street Bridge to 25 miles an hour yesterday and banned most trucks and buses from the heavily used Anacostia River crossing after cracks were detected in steel bars near the middle of the span.

Officials said the cracks, found in a relatively small number of supporting bars, do not pose a hazard.

The traffic restrictions, which took effect at midnight, were designed to lessen vibration and prevent further deterioration of the movable steel grid at the center of the bridge, officials said.

The speed limit on the five-lane structure, officially called the Frederick Douglass Memorial Bridge, previously was 35 mph.

The span, normally used by about 74,000 cars, trucks and buses daily, will be closed to heavy vehicles, including Metrobuses and trucks with a cargo capacity exceeding 1 1/4 tons.

Officials said that temporary measures have been taken to bolster the cracked parts of the bridge with steel plates. Major repairs are expected to start in about six months, they said.

The restrictions are expected to cause delays of five minutes or more for thousands of Metrobus riders. Metro officials said that more than 200 buses serving the District and Prince George's County will be required to detour across the 11th Street Bridge.

City officials said that most trucks also will have to shift to the 11th Street crossing. Small pickup trucks and vans may continue using the South Capitol Street Bridge, they said.

The D.C. Fire Department ordered all fire engines, trucks, rescue squad vehicles and ambulances to avoid the South Capitol Street Bridge except in some emergencies.

"We simply want to help," said Deputy Chief Ray Alfred, the department's spokesman.

The cracks were found in what is known as the swing span, a section in the middle of the bridge that may be rotated to allow ships to pass.

The deterioration appeared near the bridge's grid-like surface, officials said. No damage was detected in steel girders that support the grid.

Gary A. Burch, chief of design and engineering for the D.C. Department of Public Works, said the cracks were found during a routine inspection last week in bars about 5 inches thick. They appear limited to small areas of the grid, he said, and probably resulted from metal fatigue caused by vibration.

A $2 million overhaul, planned during the next two years, will be accelerated to begin shortly, Burch said.

According to Metro officials, bus detours are expected on routes A7, A8, A9, C11, D12, P5, P9, P17, P19, S12, V1, V3, W3, W11, W12, W13, W15 and W17.