The Alexandria City Council, acting in the name of fiscal responsibility, voted unanimously last night to make sweeping changes in the pension plan for the city's police and firefighters, who have threatened to challenge the move in court.

"The question is how to finance a pension-retirement program that has gotten out of control for whatever reason," Vice Mayor James P. Moran Jr. said before he voted. "If we don't do something now, we are at fault."

The council members essentially adopted the recommendations of a council subcommittee last month. The committee, of which Moran was a member, concluded that the city's disability plan was too liberal and that if its eligibility requirements, among other details, could be tightened, the city could save as much as $500,000 this year.

The council sliced precisely that sum from the pension program when it adopted the city's new operating budget. Council member Donald C. Casey said yesterday that last night's action was "set in motion" by the budget cut.

Among the changes, the council's action reduces all future service-connected partial disability benefits from slightly more than 66 percent of a public safety employe's final average earnings to 50 percent. The new plan will also trim cost-of-living adjustments for some retired workers from 5 percent to 3 percent and abolish them until age 60 for partially disabled workers.

Major elements of the amended plan are provisions that police and firefighters seeking partial disability be unable to perform any jobs they would be qualified for, as determined by their administrators. Several council members have said they believe the partial disability program has been abused by workers who retire and then secure other jobs as stressful as the ones they left while continuing to receive tax-free benefits from the city.

As the council explained its position, about 200 police and firefighters who packed the council chamber stood and either crossed their arms or raised placards in defiance.

Many grumbled and some walked out as it became apparent the measure would be approved.

"I've been the victim of larceny before, but when it happens in your own house, that adds another dimension to it," said police spokesman Sgt. Bruce Pray. "They have just taken money out of our pockets."

Richard Wohltman, an attorney representing the police and firefighters, called the council's action illegal, but would not say whether the workers would go to court to challenge it as they have threatened.