Washington DC - A US federal judge on Tuesday temporarily blocked a Louisiana law that would require the display of the Ten Commandments in every public school classroom in the conservative southern state.

District Judge John deGravelles said the law was unconstitutional and a violation of the First Amendment of the US Constitution.

The separation of church and state is a founding US principle, and the First Amendment forbids the establishment of a national religion or the preference of one religion over another.

The Louisiana law mandates the display of the Ten Commandments from January 1, 2025, in all public school classrooms, from kindergarten through state-funded universities.

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It requires the biblical text to be displayed as a poster or a framed document "in a large, easily readable font."

"If you want to respect the rule of law, you gotta start from the original law given – which was Moses," Louisiana's Republican governor, Jeff Landry, said at the bill's signing ceremony in June.

DeGravelles's ruling comes after parents of Louisiana schoolchildren and the American Civil Liberties Union (ACLU) filed suit against the Ten Commandments bill.

"This ruling should serve as a reality check for Louisiana lawmakers who want to use public schools to convert children to their preferred brand of Christianity," Heather Weaver, an ACLU attorney, said in a statement.

"Public schools are not Sunday schools, and today's decision ensures that our clients' classrooms will remain spaces where all students, regardless of their faith, feel welcomed," Weaver said.

Louisiana's attorney general, Liz Murrill, said she would appeal the district judge's ruling.

The Louisiana bill is the first of its kind to be signed into law although similar legislation has been drafted and put forward in other states in the US "Bible Belt."

In a similar case in 1980, the US Supreme Court ruled that the display of the Ten Commandments in schools in Kentucky was unconstitutional.

In Oklahoma, the highest education official in the conservative state recently ordered public schools to teach the Bible, a move that is also facing legal challenges.