Refusing to budge from its ban on prayer in public schools, the Supreme Court yesterday struck down a Louisiana law allowing children to start each school day with a voluntary prayer.

The justices affirmed, without comment, a lower court ruling that the law violated the Constitution's ban on establishment of religion by a state.

Louisiana's law provided that up to five minutes at the start of each school day could be used for offering a voluntary prayer in the classroom. Under a plan adopted by the Jefferson Parish school board, only children who verbally requested to participate in the prayer, and whose parents signed a written consent, could do so.

Citing the Supreme Court's landmark decisions in 1962 and 1963 forbidding public school prayer, the appeals court held the law was an unconstitutional entanglement of church and state.

In December, the Supreme Court refused to hear arguments in a similar New York case in which a group of high school students were banned from holding voluntary prayer meetings in a classroom before or after school hours.

The issue of school prayer is one of the items on the so-called "social agenda" now before Congress. The Senate is considering a House-passed bill that would permit voluntary prayer in schools. Efforts also are under way to bar federal courts from considering such issues as school prayer or busing to achieve desegregation.

The court also agreed to step into a voting rights dispute over a plan by Port Arthur, Tex., to revise its system of electing city commissioners.

The justices will review a federal court ruling that found the effect of the change on black residents' ballot power was not sufficiently neutralized by a new voting plan submitted by the city and the Justice Department.