A federal judge in Wisconsin ruled on Thursday that the country's National Day of Prayer is unconstitutional because it calls on citizens to take part in religious activity.

Senior U.S. District Court Judge Barbara B. Crabb said the statute that created the National Day of Prayer violates the Constitution's prohibition against the government establishment of religion.

Judge Crabb, a 1979 appointee of President Carter, wrote in her decision that "some forms of 'ceremonial deism,' such as legislative prayer, do not violate the establishment clause." But she said the National Day of Prayer goes too far.

"It goes beyond mere acknowledgment of religion because its sole purpose is to encourage all citizens to engage in prayer, an inherently religious exercise that serves no secular function in this context," she said. "In this instance, the government has taken sides on a matter that must be left to individual conscience."

In her ruling, the judge said she understood that many might disagree with her conclusion and some could view it as a criticism of prayer or those who pray.

"That is unfortunate," she said. "A determination that the government may not endorse a religious message is not a determination that the message itself is harmful, unimportant or undeserving of dissemination. Rather, it is part of the effort 'to carry out the Founders' plan of preserving religious liberty to the fullest extent possibly in a pluralistic society.'"

The ruling will not stop President Obama from recognizing, as he did last year, a national day of prayer, said White House spokesman Ben LaBolt. Judge Crabb delayed the imposition of her ruling pending an appeal.

"We have reviewed the court's decision, and it does not prevent the president from issuing a proclamation," Mr. LaBolt said.

The judge's ruling came in a lawsuit originally filed in 2008 by the Wisconsin-based Freedom From Religion Foundation. The atheist, agnostic and nontheist group said Thursday they were surprised and disappointed that the president intended to issue the proclamation.

"President Obama is a constitutional scholar, and knows the issues at stake," said the foundation Co-President Annie Laurie Gaylor.

The American Center for Law and Justice, a conservative organization that filed briefs in the case on behalf of 31 members of Congress, called Judge Crabb's ruling "flawed" and vowed to file an appeal.

"It is unfortunate that this court failed to understand that a day set aside for prayer for the country represents a time-honored tradition that embraces the First Amendment, not violates it," said Jay Sekulow, chief counsel of the ACLJ.

"If the appeals court fails to reverse this decision, we are confident the Supreme Court will hear the case and ultimately determine that such proclamations and observances like the National Day of Prayer not only reflect our nation's rich history, but are indeed consistent with the Establishment Clause of the First Amendment," Mr. Sekulow said.

Rep. Lamar Smith, a Texas Republican who is among the members of Congress for whom the amicus brief was filed, said "it is regrettable that a federal judge has called the simple national recognition of prayer - which imposes no duties or burdens on any American - unconstitutional.

"The decision undermines the values of religious freedom that America was founded upon," he said. "What's next? Declaring the federal holiday for Christmas unconstitutional?"

The lawsuit originally targeted then-President George W. Bush and members of his administration, but Mr. Obama is now listed as the defendant because the president enforces the statute in question by issuing a proclamation each year declaring National Day of Prayer.

Because proclamations for National Day of Prayer are released through the White House press office, Mr. Bush's former press secretary, Dana Perino, was originally listed as a defendant, but has since been replaced as a defendant by Robert Gibbs, Mr. Obama's press secretary.

The Justice Department, which represents the president and Mr. Gibbs in the case, would say only Thursday that it is "reviewing the judge's decision."

Every president beginning with President Truman in 1952 has issued a proclamation for the National Day of Prayer, which, since 1988, has been held on the first Thursday in May.

Last year, Mr. Obama issued a proclamation; Mr. Bush held a National Day of Prayer event at the White House in 2008.

This year's National Day of Prayer is scheduled for May 6.

The Americans United for Separation of Church and State, a religious watchdog group based in Washington, D.C., praised the ruling, calling it a "tremendous victory" for religious liberty. In a statement, it said Congress has no business telling Americans when or how to pray.

"The Constitution forbids the government to meddle in religious matters," said the Rev. Barry W. Lynn, the organization's executive director.

"Decisions about worship should be made by individuals without direction from elected officials. That's what freedom is all about.”