A federal judge yesterday invalidated the D.C. police department's policy of allowing officers in unmarked cars with no emergency lights or sirens to engage in high-speed pursuit of traffic violators. The judge said the practice is unconstitutional and dangerous.

Lawyers involved in the case said the ruling, by U.S. District Judge Royce C. Lamberth, appears to be unprecedented in addressing the use of unmarked police cars for pursuits.

Lamberth said in his ruling that motorists being chased by unmarked cars, not knowing they were being pursued by police, would be justified in fearing for their safety and would be left with the apparently unsafe options of fleeing or stopping.

Lamberth's ruling comes from a case involving a driver suspected of a traffic violation who was chased by an unmarked police car at 10 p.m. on Sept. 2, 1986. The driver, Mary L. Wright admitted in court that she ran several red lights and exceeded the speed limit during the chase, but said she did so because she didn't know her pursuers were police officers and she was afraid of them. Officers contended they pursued Wright because she was driving recklessly.

A trial is scheduled to begin next week to determine to what degree Wright and three passengers were damaged emotionally and what monetary compensation they should receive.

Lamberth set the stage for that trial by declaring yesterday that police policy allowing pursuits in such a manner is a violation of Fourth Amendment protection against unreasonable search and seizure.

"While the District may reasonably have some cars totally unmarked for undercover work, there is no governmental interest in using these cars to pursue garden-variety traffic violators," Lamberth wrote.

A spokeswoman for the District's Office of Corporation Counsel, which represents the police department, said no decision has been made on whether to appeal Lamberth's ruling. A spokesman for the D.C. police said officials had not reviewed the decision and could not comment.

Linda Delaney, Wright's lawyer, praised the ruling, saying it would protect civilians and police officers.

"As soon as someone knows they are being chased by police, they are likely to stop," Delaney said.

According to court records and interviews, Wright was driving a van in Northwest Washington and made a right turn at Georgia Avenue and Euclid Street, near four police officers in an unmarked car. Words debating the efficiency of the turn were exchanged between Wright and the officer driving the unmarked car, Norbert Savoy, according to court records.

Savoy made a U-turn and started chasing Wright.

Wright drove to her home on Sherman Avenue NW and stopped. Savoy stopped the unmarked police car in front of the van, and several officers jumped out with guns drawn, according to court records.

A shot was fired from Savoy's gun and went through the van's windshield, just missing Wright, according to Delaney and court records. Bob Deso, the lawyer for Savoy, said the gun discharged accidentally.