Walter F. Mondale yesterday accused President Reagan of tampering with the wall separating church and state and said his administration had "opened its arms" to a "determined band" of religious groups that "are reaching for government power to impose their own beliefs on others."

In his sharpest speech on the role of religion in politics, he chided Republicans for clothing support for the president's reelection in religious garb.

"Most Americans would be surprised to learn that God is a Republican," the Democratic presidential nominee noted to scattered laughs and applause from 1,500 delegates here at the international convention of B'nai B'rith, the Jewish service organization.

He repeated much of the speech later in the day before the predominantly black National Baptist Convention and coupled it with a sharp attack on Reagan's civil rights record.

Reagan, speaking to B'nai B'rith two hours after Mondale, did not respond directly to his challenger. But in a speech that dealt mainly with U.S.-Israel relations, he inserted a strong call for religious pluralism and tolerance.

"The unique thing about America is a wall in our Constitution separating church and state," Reagan said.

"It guarantees there will never be a state religion in this land but at the same time it makes sure that every single American is free to choose and practice his or her religious beliefs or to choose no religion at all. Their rights shall not be questioned or violated by the state."

Reagan had sounded a different note at a prayer breakfast on the final day of the Republican National Convention in Dallas last month, stating that religion and politics are "necessarily related" and that opponents of a constitutional amendment for school prayer are "intolerant" of religion.

Those comments "really got Mondale's juices going," in the words of one aide, and in the past 10 days the Democrat has four times blasted what he described yesterday as the "moral McCarthyism" of "extreme" forces in the Republican Party who call into question the religious faith of those who disagree with them on public policy matters.

Despite the heavy campaign rhetoric about religion in the past few weeks, strategists for both camps insist that it is an issue that they are not eager to keep alive.

Reagan strategists believe that they have gotten all the support they will get from the fundamentalist Christians who emerged as a strong voting bloc in 1980, and they fear that they could suffer a backlash among young liberal voters who are turned off by the perceived intolerance of Rev. Jerry L. Falwell and others.

Mondale strategists characterized the speech yesterday as an effort to close off, rather than prolong, discussion of what they believe to be a sensitive issue.

"What I am doing here today is something that, in 25 years of public life, I never thought I would do: I have never before had to defend my religious faith in a political campaign," Mondale, the son of a minister, said.

"I have never thought it proper for political leaders to use religion to partisan advantage by advertising their own faith, and question their opponent's . . . . "

"No president should attempt to transform policy debates into theological disputes. He must not let it be thought that political dissent from him is un-Christian. And he must not cast opposition to his programs as opposition to America."

While Mondale criticized Reagan for attacking "those of us who are trying to preserve the constitutional separation of church and state," he reserved stronger criticism for some of the president's political and religious associates.

He described as "partisan zealotry" a letter from Sen. Paul Laxalt (R-Nev.), chairman of Reagan's reelection campaign, to 45,000 ministers in which Laxalt described Reagan's supporters as "leaders under God's authority."

And he said it is "troubling" that television evangelist Rev. Jimmy Swaggart, "who insists that Catholicism is a 'false religion' and that Jews are damned to go to hell, is a welcome policy adviser in the White House."

"There is no more uplifting power on earth than a religious faith that cannot be coerced and is tolerant of other beliefs," Mondale said.

"To coerce it is to doubt the sturdiness of our faith. To ask the state to enforce the religious life of our people is to betray a telling cynicism about the American people.

"Moreover, history teaches us that if that force is unleashed, it will corrupt our faith, divide our nation and embitter our people. Today that force is being wielded by an extreme fringe poised to capture the Republican Party and tear it from its roots in Lincoln."

Later yesterday, Mondale touched similar themes in a speech for thousands of ministers at the National Baptist Convention, where he was introduced by Jesse L. Jackson.

It was the first joint appearance by the two since Jackson endorsed Mondale following a meeting last week in North Oaks, Minn.

Mondale drew his best response when he told the audience that the president was right when he recently told supporters of a possible second term: "You ain't seen nothing yet."

"When you look at his administration, when it comes to civil rights, we ain't seen nothing at all," Mondale said, and followed with a litany of issues ranging from social security to school lunch programs to student loans to tax fairness. He said Reagan's record had been dismal on all fronts and added that if the country reelects the president, "You ain't seen nothing yet."

Before the Baptist ministers, he repeated his call for a strict separation of church and state, an issue that has a somewhat different spin with the black clergy, where there is a long tradition of involvement in both social causes and partisan activity.

The National Baptist Convention refrained from endorsing candidates, but many black Baptist ministers in the group do so individually.

In his speech to B'nai B'rith, Mondale concluded by saying that while he stood for a return to traditional values, he also cautioned, "Family must not become a code for intolerance. Religion must not become code for censorship. Neighborhood must not become code for discrimination. Law must not become code for repression. Work must not become code for callousness. Flag must not become code for jingoism. Peace must not become code for war."