Nov 12 (Reuters) - A federal judge on Tuesday declared unconstitutional a Louisiana law requiring that the Ten Commandments be displayed in all public school classrooms in the state.

The decision by U.S. District Judge John deGravelles is a temporary setback for conservative groups trying to make expressions of faith more prominent in society.

Schools would have been required to comply with the law by Jan. 1. The judge was appointed to the bench by Democratic President Barack Obama.

Liz Murrill, the state's Republican attorney general, said in a statement: "We strongly disagree with the court's decision and will immediately appeal."

An appeal would go to the 5th U.S. Circuit Court of Appeals in New Orleans, widely considered among the most conservative federal appeals courts.

In his 177-page decision, deGravelles said Louisiana's law conflicted with a 1980 U.S. Supreme Court decision voiding a similar law in Kentucky, and violated the religious rights of people who opposed the displays.

He also said the law was "discriminatory and coercive" because it would pressure children in public schools into adopting Louisiana's preferred religious teachings while attending school at least 177 days per year.

"Each of the plaintiffs' minor children will be forced in every practical sense, through Louisiana's required attendance policy, to be a 'captive audience,'" the Baton Rouge-based judge wrote.

"The issue is whether, as a matter of law, there is any constitutional way to display the Ten Commandments in accordance with [Louisiana's law],'" he added. "In short, the court finds that there is not."

NO 'BROADER TRADITION'

Louisiana became the only U.S. state requiring displays of the Ten Commandments when Republican Governor Jeff Landry signed House Bill 71 into law on June 19.

The law requires the display of posters or framed versions of the Ten Commandments that are at least 11 inches by 14 inches, with the Commandments being the "central focus" and printed in a large, easy-to-read font.

Nine families, including several clergy, with children in public schools sued five days later in Baton Rouge, Louisiana's capital, seeking an injunction against the law.

The various plaintiffs are either Unitarian Universalist, Jewish, Presbyterian, nonreligious or atheist.

"H.B. 71 is a direct infringement of our religious-freedom rights," said Darcy Roake, a Unitarian Univeralist minister who with her Jewish husband Adrian Van Young is among the plaintiffs, in a statement.

"As an interfaith family, we expect our children to receive their secular education in public school and their religious education at home and within our faith communities, not from government officials," Roake added.

Some conservatives hope the Supreme Court's 6-3 conservative majority would uphold such laws against legal challenges.

The court sided in 2022 with a Washington state high school football coach who claimed a constitutional right to pray with his players at the 50-yard line after games.

Judge deGravelles said Louisiana's law was unconstitutional under that decision as well, because there was no "broader tradition" of using the Ten Commandments in public education.

The case is Roake et al v Brumley et al, U.S. District Court, Middle District of Louisiana, No. 24-00517.

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Reporting by Joanthan Stempel in New York; Additional reporting by Brendan O'Brien in Chicago; Editing by Jonathan Oatis and Deepa Babington