The Freedom From Religion Foundation sued in the US district court, arguing that the president's mandated proclamations calling on Americans to pray violates a constitutional ban on government officials endorsing religion.

The day of prayer, held each year on the first Thursday of May, creates a "hostile environment for non-believers, who are made to feel as if they are political outsiders," the lawsuit said.

The national proclamation issued this year asked God's blessings on our country and called for Americans to observe the day with appropriate programmes, ceremonies and activities.

Wisconsin Governor Jim Doyle is named in the suit because he is one of 50 governors who issued proclamations calling for the prayer day. The foundation is based in Madison, Wisconsin.

Shirley Dobson, chairwoman of the National Day of Prayer Task Force, and White House press secretary Dana Perino are also named.

The foundation has filed numerous lawsuits in recent years, including one rejected by the US Supreme Court last year that attacked President Bush's faith-based initiative.

The White House and a spokesman for Governor Doyle had no comment on the lawsuit.