MARK SHERMAN Associated Press Writer

(AP) - WASHINGTON-A half-century after the Supreme Court outlawed racially segregated schools, sharply divided justices clamped new limits on local school efforts to make sure children of different races share classrooms.

The top U.S. court voted 5-4 Thursday to strike down school integration plans in Louisville, Kentucky, and Seattle, a decision that imperiled similar plans that hundreds of cities and counties use voluntarily to integrate their schools.

The ruling does not affect several hundred other public school districts that remain under federal court order to desegregate.

Justices disagreed bluntly with each other in 169 pages of written opinions on whether the decision supports or betrays the landmark Brown v. Board of Education ruling that led to the end of state-sponsored school segregation in the United States.

The 5-4 decision, the 24th such split this term, displayed the new dominance of the court's aggressive conservative majority. The four liberal justices dissented.

Chief Justice John Roberts asserted in his majority opinion that by classifying students by race, the school districts are perpetuating the unequal treatment the Brown decision outlawed. "The way to stop discrimination on the basis of race is to stop discriminating on the basis of race," Roberts said.

Citing Brown to rule against integration was "a cruel irony," responded Justice John Paul Stevens in his dissent.

Crucially for school districts seeking guidance, Justice Anthony Kennedy went along with the court's four most conservative members in rejecting the Louisville and Seattle plans but also said race may sometimes be a component of school efforts to achieve diversity.

To the extent that Roberts' opinion could be interpreted to foreclose the use of race in any circumstance, Kennedy said, "I disagree with that reasoning."

"A district may consider it a compelling interest to achieve a diverse student population," Kennedy said. "Race may be one component of that diversity."

Kennedy seemed to suggest that race could be a factor in deciding where to build a new school or how to draw school attendance boundaries.

Justice Stephen Breyer, in a pointed dissent he read in the courtroom, said those measures have had only limited success in promoting integration.

Breyer was more expressive than usual in the elegant courtroom, grimacing a time or two, shaking his head and rolling his eyes as Roberts read from his opinion.

Joined by the other liberals on the court, Breyer said Roberts' opinion undermined the promise of integrated schools that the court laid out 53 years ago in the Brown decision. "To invalidate the plans under review is to threaten the promise of Brown," he said.

On the other side, Justice Clarence Thomas, the court's only black member, wrote a separate opinion endorsing the ruling and taking issue with the dissenters' view of the Brown case.

"What was wrong in 1954 cannot be right today," he said. "The plans before us base school assignment decisions on students' race. Because 'our Constitution is colorblind and neither knows nor tolerates classes among citizens,' such race-based decisionmaking is unconstitutional."

Civil rights leaders, trying to make the best of the decision, said Kennedy's opinion, when combined with the four dissenters, showed that a majority of the justices support the continuing use of race-conscious measures to integrate public schools.

"We got rained on today, but there's a silver lining," said Theodore Shaw, president of the NAACP Legal Defense and Education Fund.

Dennis Parker, director of the American Civil Liberties Union's Racial Justice Program, said, "Even so, the rejection of the Seattle and Louisville school plans represents a significant step backwards in a nation where schools are becoming increasingly segregated by race and ethnicity."

School districts that have plans that resemble the ones struck down by the court are expected to look for other ways to make their schools racially balanced without specifically relying on race. One possibility is using family income since blacks are more likely than whites to be poor.

The Louisville case grew out of complaints from several parents whose children were not allowed to attend the schools of their choice. Crystal Meredith, a white, single mother, sued after the school system turned down a request to transfer her 5-year-old son Joshua Ryan McDonald, to a school closer to home.

The Seattle school district said it used race as a factor only at the end of a lengthy process in allocating students among the city's high schools. Seattle suspended its program after parents sued.

Kathleen Brose, mother of a white Seattle student who sued the district, said she felt vindicated by the decision. "We've never said we didn't like diversity," she said. "We're against discrimination. ... There's just other things they can do without discriminating."