Iran turned up the volume of its anti-American rhetoric again today in heralding a three-day international conference staring Monday designed to prove that the United States is responsible for crimes and the plundering of our resources for the past 17 years."

The conference was called by President Abol Hassan Bani-Sadr after the aborted U.S. attempt April 25 to rescue 53 American hostages held since November.

About 400 persons representing Third World nations and liberation movements are expected to attend the conference, Iranian officials said, but they refused today to list any of the participants.

(A delegation of Americans, including former attorney general Ramsey Clark and Princton professor Richard Falk, had gathered in New York on Friday night to prepare to travel to the conference. But they delayed their departure after the Justice Department, citing President Carter's ban on travel to Iran, threatened them with 10-year jail terms and $50,000 fines.

(A State Department spokesman said today in Washington that the American delegation's presence in Tehran would be used for propaganda purposes and "misconstrued" as U.S. acceptance of the conference's "claim that the United States has committed aggression against Iran."

[It was not clear late tonight whether the group would defy the travel ban. One member of the delegation said the group would announce its intentions in a statement Sunday.]

Iranian officials said the delegation that was blocked from traveling was made up of Americans "who have proved their anti-imperialist credentials."

"This threat proves the U.S. has no intention of letting the people of the United States and the people of Iran establish a dialogue. It also demonstrates that a policy of subversion and intervention by the U.S. in Iran has not come to an end," said Ahmad Salamatian, an aide to Bani-Sadr and secretary general of the conference.

The conference comes at a time when there appears to be distinct toning down of the anti-American rhetoric here. At a massive Friday prayer meeting, for example, there were none of the cries of "america the great satan" or "that devil Carter" or descriptions of the U.S. Embassy here as "the nest of spies" that marked similar meetings five months ago, during the height of the national hysteria over the hostage-taking.

Moreover, the keynote speeches at the opening of the newly elected Iranian parliament, the Ajlis, made no mention of the hostages or of American and instead concentrated on Rian's domestic problems.

"People still feel as hostile to the United States as when the students captured the embassy last year," an aide to Bani-Sadr commented last week. "But they worry more these days about inflation, unemployment, lack of security and trouble with Kurdistan."

A poll of 31 of the Majlis' 270 representatives published today in the daily newspaper Kayhan showed the return of the hostages stood low on a list of priorities. Most of those polled were members of the hard-line clerical "Islamic Republican Party." They said the hostages should not be released unless the United States turns over the deposed shah and his wealth to Iran. Many favored putting the hostages on trail before any release.

Only one of the delegates polled, Dr. Ahmad Beheshti of Fasa, expressed any sympathy for the quick return of the hostages. "On the one hand," he said, "we have not reached the conclusions we had hoped from the hostage taking. On the other hand, keeping these hostages has incurred expenses and excuses for us."

The leader of the Islamic Republican Party, Ayatollah Mohammed Beheshti, said, "We have many more problems than the hostages to think about."

Members of the parliament -- who were given the responsibility of settling the hostage crisis by Iran's political and religious leader, Ayatollah Ruhollah Khomeini -- have been invited to attend "the international conference on U.S. intervention in Iran" as a way of familiarizing themselves with the investigation of America's "crimes against Iran."

The conference is to open with a report prepared by authorities here on the "interv entionist and criminal activities of the United States in Iran during the past 27 years in order to demonstrate the historical context and the roots of the present confrontation between the two countries," Salamatian said.

Iran has maintained that taking the U.S. hostages Nov. 4 was justified by the strong American support of the deposed shah, including putting him back in power in 1953 in CIA-engineered coup. Moreover, Iranian officials long have complained that the United States views the hostage issue in isolation instead of connecting it to what is seen here as a long history of U.S. interference in Iranian affairs.

"We wish to prove," Salamatian said, "that the U.S. did not have normal diplomatic relations with Iran but was acting as a power determining this country's policies and destinies according to the U.S., not Iranian, interests.

"We hope participants in this conference will decide for themselves where the international legitimacy lies and the righteousness of Iran's position," he said.

While Salamatian refused to say who would be attending the conference, it is believed that most of the delegations will be from "liberation movements" that have opened offices here.