President Carter, emphasizing his role in directing the effort to free the U.S. hostages in Iran, last night canceled his scheduled two-day trip to Canada and had Secretary of State Cyrus R. Vance call for "the continued support of the American people" in the lengthening crisis.

Vance's statement, which he read to reporters, was described by administration officials as a high-level attempt to calm rising national concern and anger about the hostages' plight.

One official said it was intended "as an across-the- board appeal to political candidates, members of Congress, the press and the general public" to avoid actions or statements that might inflame the situation and make more acute the danger to the hostages.

"The situation is extremely difficult and delicate," Vance said. "I am sure that all Americans understand that the efforts we are pursuing cannot take place in the glare of publicity. Let me assure you, however, that we are pursuing every avenue open to us to secure their safe and early release. Our actions will continue to be guided by that overriding objective."

The secretary's statement was followed shortly thereafter by a White House announcement that Carter, who had been scheduled to visit Canadian Prime Minister Joe Clark in Ottawa today and tomorrow, was postponing the trip until sometime next year. White House press secretary Jody Powell said Carter wanted to remain here in close contact with his foreign policy advisers.

Earlier in the day, Carter, in welcoming Irish Prime Minister Jack Lynch to the White House, alluded indirectly but unmistakeably to the situation in Tehran and to Ireland's problems with terrorism.the president told Lynch:

"We both have experienced the adverse consequences of terrorism, of murders perpetrated and murders threatened against innocent people. Those who advocate violence and terrorism violate the laws of God, the laws of man."

In the meantime, the administration, frustrated at every turn by events in Iran, continued to stress that it is not contemplating any military action to replace its strategy of trying to free the hostages through some form of diplomatic negotiations.

But, what appeared early yesterday to be the most potentially promising channel to the Iranians -- intercession by the Palestine Liberation Organization -- was dampened considerably last night by reports from Tehran that the revolutionary students holding the hostages were refusing to negotiate with the PLO.

Still, the United States, in the most open approach it ever has made to the PLO, authorized President Carter's special envoy, Ramsey Clark, to talk with PLO officials in Istanbul, where Clark has been marking time while trying to get permission to enter Iran.

"He has been told by us to hold whatever conversations he believes are necessary to facilitae the rapid andsafe release of our embassy people," State Department spokesman Hodding Carter said. Other department officials confirmed privately that Clark's talks included contacts with the PLO.

Clark is carrying a letter from President Carter to Ayatollah Ruhollah Khomeini calling for the prompt release of the hostages held in the U.S. embassy. Officials said the letter suggests that the Americans promptly leave Iran, but denied that any decision has been made to terminate diplomatic relations with Iran.

Although the PLO's role in the situation was clouded by conflicting statements made in different places, the mere suggestion of its involvement has posed some delicate problems for theadministration. U.S. policy forbids direct negotiating or contact with the PLO until it recognizes Israel's right to exist.

To get around that problem, Hodding Carter and other U.S. officials cited what they called the "precedent" of 1976 when the PLO helped evacuate Americans from Beirut as the Lebanese capital was swept by civil war.

However, the PLO's role in the 1976 Beirut crisis was carried out on an almost sub rosa basis with little public discussion by either party. In contrast, the State Department yesterday volunteered a statement to reporters that later was repeated by Hodding Carter. It said:

"If they (the PLO) are moving to help release the Americans, it would be a highly responsible action in a situation where they might have some influence, and we welcome such assistance."

As of last night, though, whether the PLO could prove helpful as a mediating force was unclear. Despite reports from Tehran that the students holding the U.S. embassy had rejected talks with the PLO, Hatem Hussaini, head of the PLO information office here, told a press conference that PLO representatives and Iranian officials had "extensive consultations in Tehran" and that "they welcomed our ideas and views."

The rise in public anger was evident in a tide of telephone calls to the White House and Capitol Hill, and several demonstrations and displays throughout the country.

Several hundred angry demonstrators surrounded the main entrance of the Iranian consultate in Houston, chanting, "take the oil and shove it" and burning two Iranian flags. This evidently was in reaction to a lunchtime march the day before by Iranian students supporting the Tehran authorities.

There also were confrontations between Iranian students and Ameri- cans in Portland, Ore., and Carbondale, ill. Opposing groups traded taunts and some punches but there were no reports of injuries.

In New York, the president of the AFL-CIO Transport Workers Union, William Lindner, said union members would stop servicing Iranian aircraft at U.S. airports until the hostages are freed.

An Iran Air flight to New York was diverted to Montreal when union members refused to unload the plane.

On Capitol Hill, Deputy Secretary of State Warren Christopher appealed to an overflow meeting of several hundred House members to exercise restraint in their public statements to avoid jeopardizing the lives of the American hostages. Lawmakers told Christopher they were being bombarded with calls from indignant constituents demanding drastic U.S. retaliatory steps.

The American Foreign Service Assoication, made up of U.S. diplomatic personnel, urged curbs on public utterances in a statement sent to Carter and to all presidential candidates. The group noted that two U.S. diplomats held hostages in the Sudan in 1973 were executed after "provocative" remarks were made in Washington. Despite their own intense feelings and the public demand for strong talk and action, political leaders for the most part heeded the pleas for restraints in their rhetoric.

In the Senate, Gary Hart (D-Colo.) called for the United States to reduce its energy consumption by 10 percent to preempt the threat of an Iranian oil cutoff. His proposal came as spokesmen for major U.S. oil companies reported that Iran is cutting its fourth quarter deliveries to the majors by about 5 percent, a much smaller amount than had been feared.

A total of 226 members of the House, headed by Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), signed a letter to Ayatollah Ruhollah Khomeini which was delivered late Wednesday to the Iranian embassy here. The letter called for the prompt release of the American hostages "in the spirit of justice" and suggested that this act could establish a climate for mutually beneficial relations between Iran and the United States.

Rep. G. William Whitehurst (R-Va.) introduced legislation to require the deportation of foreign students whose home nations engage in or permit hostile actions against U.S. citizens.

An official said yesterday that the g overnment has decided to halt shipments of military equipment spare parts to Iran for the time being.

In New York, a spokesman for the deposed Shah Mohammad Reza Pahlavi issued a brief statement saying that the shah has "expressed his willingness" to leave the United States in hopes the American hostages would be freed but that "his physicians remain adamant in their position that a patient in his condition cannot possibly leave without risking his life."

The shah was described as recovering from surgery but "now somewhat feverish." The statement said a further gallstone operation is expected in three weeks, and that his "expanding tumor of the neck" will be treated with radiation.