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

The court voted 5-4 to strike down school integration plans in Louisville, Ky., 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 ruling also could unsettle the more than 2,000 magnet schools that educate 2 million children since many were created under desegregation plans, said UCLA education expert Gary Orfield.

"These are some of the only integrated things in a lot of our inner cities," Orfield said. "It's one of the only ways to keep middle-class kids involved in big-city school systems."

The Jefferson County and Seattle school systems, whose integration plans were the subjects of Thursday's decisions, employ slightly different methods of taking students' race into account when determining which schools they will attend.

Federal appeals courts had upheld both plans after some parents sued. The Bush administration took the parents' side, arguing that racial diversity is a noble goal but can be sought only through race-neutral means.

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.

Louisville's schools spent 25 years under a court order to eliminate the effects of state-sponsored segregation. After a federal judge freed the Jefferson County, Ky., school board, which encompasses Louisville, from his supervision, the board decided to keep much of the court-ordered plan in place to prevent schools from re-segregating.

Louisville Mayor Jerry Abramson said he was disappointed with the ruling because his city's system had provided "a quality education for all students and broken down racial barriers" for 30 years.

Deborah Stallworth, a Louisville parent who successfully sued to end court-ordered busing in 2000, said: "We send children to school to be educated, not as a social experiment."

Attorney Teddy Gordon, who argued that the Louisville system's plan was discriminatory, said Thursday, "Clearly, we need better race-neutral alternatives. Instead of spending zillions of dollars around the country to place a black child next to a white child, let's reduce class size. All the schools are equal. We will no longer accept that an African-American majority within a school is unacceptable."

Pat Todd, executive director for student assignment for Jefferson County Public Schools, said there would be no changes in student assignments for the coming school year.

Gordon said if the school board goes forward with the current system that uses race in assignments, he will ask a federal judge to hold the board in contempt of court for violating the Supreme Court's decision.

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."

The opinion was the first on the divisive issue since 2003, when a 5-4 ruling upheld limited consideration of race in college admissions to attain a diverse student body.

Since then, Justice Sandra Day O'Connor, who approved of the limited use of race, retired. Her replacement, Justice Samuel Alito was in the majority that struck down the school system plans in Kentucky and Washington.

Thursday's decision left in place the 2003 ruling.

The cases are Parents Involved in Community Schools v. Seattle School District No. 1, 05-908, and Meredith v. Jefferson County Board of Education, 05-915.