WASHINGTON -- The Supreme Court, handing civil rights forces a sweeping victory, yesterday reaffirmed the legality of affirmative action in the American workplace to remedy past discrimination against Blacks and other minorities.

The court's latest word on affirmative action came in two rulings.

In one, the justices approved a plan in Cleveland that reserves about half the promotions in the city's fire department for qualified minority candidates.

In the other, the court upheld a ruling that a union representing sheet metal workers in New York and New Jersey must significantly raise its non-white membership by August 1987.

The rulings represent a major defeat for the Reagan administration, which sought to limit on-the-job racial preferences.

By a 6-3 vote in the sheet metal workers case, the court rejected the administration's position that only actual victims of discrimination may benefit from court-ordered affirmative action remedies.

The justices said that more sweeping remedies do not violate federal civil rights laws or the Constitution's equal-protection guarantees.

Justice William J. Brennan, in the court's majority opinion, said: "We hold that [federal law] does not prohibit a court from ordering, in appropriate circumstances, affirmative race-conscious relief as a remedy for past discrimination."

Writing for the court's 6-3 majority in the Cleveland case, Brennan said agreements between employers and minority groups may provide racial preferences even more extensive than a federal court would have awarded after a trial.

He said such agreements do not amount to unlawful "reverse discrimination" against white males.

In today's rulings, Brennan was joined by Justices Thurgood Marshall, Harry A. Blackmun, Lewis F. Powell, John Paul Stevens and Sandra Day O'Connor.

Dissenting were Justices Byron R. White, Chief Justice Warren E. Burger, who is about to retire, and Justice William H. Rehnquist, Reagan's choice to succeed Burger as chief justice.

The White House had no immediate comment.

But Rep. Don Edwards, D-Calif., the chairman of the House civil rights subcommittee, said that the high court "recognizes that in order to get beyond racism and sexism, we often must take race and sex into consideration. It's both silly and naive to think that we live in a color-and gender-blind society that would allow the passive policies of the Justice Department to work."

Defending his department's policies, Attorney General Edwin Meese said he remains opposed to "an affirmative action process which in itself is discrimination, which in itself deals with people on the basis of their race rather than their qualifications."