President Reagan told ministers, rabbis and priests gathered in the White House Rose Garden yesterday that he intends to propose to Congress a constitutional amendment that would allow prayer in public schools.

Urging a reawakening of "America's religious and moral heart" and the protection of religion from "government tyranny," Reagan reiterated his long-held belief that the Supreme Court is wrong in consistently denying school-sponsored prayer in public classrooms over the last 20 years.

"No one will ever convince me that a moment of voluntary prayer will harm a child or threaten a school or state," Reagan said. "But I think it can strengthen our faith in a creator who alone has the power to bless America."

Reagan was careful in his remarks, as was a White House fact sheet released for the event, to stress that the proposed constitutional amendment the Justice Department is drafting would only allow voluntary prayer. Children or school districts that chose not to pray could not be forced to do so, which administration spokesmen argue would not conflict with First Amendment guarantees of freedom of religion.

At the same time yesterday, Attorney General William French Smith said he has questions about the constitutionality of a bill sponsored by Sen. Jesse Helms (R-N.C.) to deny federal courts power to rule in school prayer cases, but that if it became law the Justice Department would enforce it.

Reagan's proposal for a school prayer constitutional amendment was a gesture to conservatives who had grown increasingly restless as the White House relegated "social issues" to a back burner while emphasis was given to budget and economic battles. Restoration of prayer to schools is perhaps the least controversial of the social issues, far less than school busing and abortion.

In the two decades since the Supreme Court ruled that the state may not include prayers in public school curriculum, mainline churches and synagogues have battled a school prayer amendment just as zealously as conservative evangelicals have fought to get one enacted.

Religious leaders representing about 60 national groups, Protestant, Eastern Orthodox and Jewish, outlined their objections in a joint statement at a press conference yesterday on Capitol Hill:

The First Amendment of the Constitution prohibits public schools "from fostering religious practices or beliefs."

A least-common-denominator prayer that would be acceptable to all "trivializes prayer by robbing it of depth and meaning."

Religious instruction is the responsibility of the religious family and the religious community.

But the Southern Baptist Convention, in the vanguard of evangelicalism theologically, takes sharp exception. One of the most outspoken critics of Reagan's proposal yesterday was the Rev. James Dunn, executive director of the Baptist Joint Committee on Public Affairs.

"It is despicable demagoguery for the president to play petty politics with prayer," said Dunn. "He knows that the Supreme Court had never banned prayer in schools. It can't. Real prayer is always free . . . . What the court has done is protect religious liberty."

Proponents of a prayer amendment stress the voluntary aspect such legislation would embody. "We're on the record for voluntary school prayer," said Forest D. Montgomery, legal counsel for the National Association of Evangelicals. But he acknowledged that "it's a very delicate situation. You've got to be sensitive to all faiths and beliefs, and you've got to be sensitive to the kids. But you have to take majority rights and minority rights into account."

The Rev. Jerry Falwell, founder of the Moral Majority, circulated through the crowd afterward granting numerous interviews and promising to direct "every resource" of his organization to getting the constitutional amendment approved.

"I think it's a bright day in America," said Falwell. "After 20 years of expulsion of Almighty God from the public schools of the United States of America, I think this is the light at the end of the tunnel."

"The nine members of the Supreme Court will no longer be able to deny children their opportunity to pray in school buildings paid for by their moms and dads."

Rabbi Alexander M. Schindler, New York-based president of the Union of American Hebrew Congregations, maintains that " 'voluntary' school prayer is a misnomer. There is nothing voluntary in the school setting, where pupils are taught, and rightly, to obey the teacher and cooperate in class," he said.

Dr. Clair Randall, general secretary of the National Council of Churches, believes that "the religious training of children is the responsibility of the family and the church; it is not the responsibility of government at any level."