A hostile crowd of white demonstrators forced Walter F. Mondale and Rep. Harold Washington, the Democratic mayoral candidate, to cut short a visit to a Palm Sunday church service here today.

The crowd of more than 150 shouted epithets at the former vice president and at Washington, the first black to win a Chicago mayoral nomination, as they entered St. Pascal Catholic Church on the city's northwest side.

The demonstrators swarmed onto the church steps, booing loudly and chanting, "Epton, Epton," for Washington's Republican opponent, Bernard E. Epton, and "carpetbagger Mondale, carpetbagger Mondale."

After a few moments inside the church, Mondale and Washington decided to leave rather than allow the demonstrators to further disrupt services.

Police cleared a path down the church steps and the two men calmly left in a motorcade as demonstrators shouted, "No more politics in the church," and "Washington, you're a tax evader," an apparent reference to Washington's 1972 conviction on a charge of failing to file income tax returns for four years.

Washington aides later said it was the ugliest incident that the two-term congressman has encountered since he upset Mayor Jane M. Byrne and Cook County State's Attorney Richard M. Daley in the Feb. 22 Democratic primary.

The incident also pointed out the dangers of racial politics for both Republican and Democratic parties. The Chicago mayor's race has taken on national importance, and both parties are committing major resources.

Washington called the incident a "dastardly thing" that "desecrated" and "defamed the church," and "embarrassed a fine priest" on "one of the holiest of days."

"Mr. Epton, you have some explaining to do," he said at a labor rally of 12,000 that included AFL-CIO President Lane Kirkland and United Auto Workers President Douglas Fraser. "You have to explain . . . why you would unleash that kind of group of people who have no respect for anyone's institutions."

"It would never have happened in my community," Washington added. "We would not tolerate it."

The incident marred an otherwise successful peace-keeping mission here for Mondale, who had angered Washington and other blacks by endorsing Daley, son of the late mayor, in the primary.

Mondale, a 1984 presidential candidate, blamed the incident on Epton.

"I'm an old expert on demonstrations. I've been through thousands of them," Mondale said. "It was obvious this was all political and that the other candidate Epton was pushing that stuff. This didn't look like an indigenous, spontaneous local thing."

Epton, at a rally of his own, took offense at the remarks of Washington and Mondale. He said the church incident was "unfortunate" and "appalling," and added that his campaign had nothing to do with it. "What was more appalling is that a former vice president would suggest that I instigated that type of thing," he said.

In the traditionally Democratic ethnic neighborhood where Mondale and Washington were booed, many wore Epton buttons and carried "Epton for Mayor" signs.

"Mondale, why are you supporting Washington?" yelled Judy Hertsgaard, a lifelong Democrat. "All you want is a black vote. You don't want us whites. All the Norwegians are ashamed of you."

"I'm a Catholic and I think it's a disgrace they came here," said Vito Dizonno, who carried a "Democrats for Epton" sign. "This should have never happened at a Catholic Church. We are just a bunch of neighborhood people who are mad."

Mondale was the first of a series of Democratic heavy hitters scheduled to appear here this week with Washington, including Sen. John Glenn (Ohio), Rep. Claude Pepper (Fla.) and New York Gov. Mario Cuomo.

Washington had been so angry with Mondale's endorsement of Daley in the Democratic mayoral primary that he refused to return the former vice president's telephone calls for eight days after the contest. He appeared pleased to have Mondale at his side today. The two men, both sons of ministers, met privately early in the morning; spoke before a meeting of the American Federation of State, County and Municipal Employes and attended services at the Progressive Community Church, Washington's home congregation.

At the church, Mondale was asked to kneel alone at the altar where he was blessed by the Rev. B. Herbert Martin.

Washington acknowledged that he and Mondale have had problems in the past, but said all has been forgiven. "Democratic primaries are spirited by tradition. There is bloodletting; there are controversies," he told a news conference. "After primaries, we come together and pick up the dead and wounded. What controversy existed between Mr. Mondale and I has long since dissipated."