Twenty-seven persons were injured yesterday when a fully loaded Metrobus skidded out of control and overturned on its side during an evening rush-hour downpour on Branch Avenue in Prince George's County. The most severly injured, police said, appeared to be two persons with dislocated shoulders.

State police said the driver of the southbound Rte. C11 bus, Donna Wright, 31, said the brakes locked as she slowed for a red light at Allentown Road just before 6 p.m. Metro safety inspectors and state police investigators plan to check the brake system for defects today, police said.

About a dozen ambulances carried the injured -- most suffering cuts, bruises and sprains -- to three nearby hospitals.

"It was really amazing -- if you've ever seen that intersection during rush hour -- that no other vehicles were involved," said Maryland State Police Trooper John Fields. He said traffic was snarled for about an hour after the accident, mostly by the numerous ambulance arrivals and departures and by efforts to right the bus and tow it away.

The bus, according to witnesses, apparently skidded in a counterclockwise direction, and sliding sideways, tipped on its side onto the grassy center median. The driver, who suffered bruises, reported that several passengers climbed out of the overturned bus and ran away, Fields said. The bus was headed for Clinton from Federal Center in Southwest D.C.

A spokesman for Southern Maryland Medical Center reported that 12 passengers were treated for cuts and bruises and two for dislocated shoulders. At Malcolm Grow Hospital at Andrews Air Force Base, three were treated for minor injuries and released, and a fourth was being held for tests results, but was described as in good condition. Greater Southeast Community Hospital's emergency room treated nine patients for minor lacerations and bruises.