U.N. Secretary General Kurt Waldheim today met with hostile demonstrations and denunciations by the militants holding the hostages at the U.S. Embassy, raising doubts about whether his mediation effort will be successful in forestalling Security Council consideration Monday of economic sanctions against Iran.

The student militants at the embassy branded Waldheim's mission a "vague and suspicious trip" and referred to him as a pawn of the United States.

Waldheim met for three hours with Foreign Minister Sadegh Ghotbzadeh in the morning, but his afternoon activities were canceled abruptly after a large, unruly crowd demonstrated against him and the government news agency announced that an alleged death plot against the secretary general had been thwarted.

Waldheim's inactivity on his first full day in Tehran provided further indication that his efforts were unlikely to obtain release of the hostages by Monday. The Security Council has threatened economic sanctions against Iran unless the Americans are freed by that day.

Waldheim warned after his arrival here yesterday that his mission would not "solve immediately all problems." He is believed to hope for little more than to establish a climate of trust with the suspicious Iranian leaders within the next several days so that he can report some progress to the Security Council in hopes of extending the deadline.

The diplomatic stalemate in Tehran continued to be accompanied by unrest in Iran's provinces.

Tehran Radio reported that clashes last night in western Sanandaj had brought the Kurdish provincial capital to a standstill.

Three people were reported dead and another eight injured, it said. An estimated 5,000 Sanandaj residents reportedly staged a sit-in at the offices of the Kurdestan governor general to demand the expulsion of the Revolutionary Guard corps loyal to Ayatollah Ruhollah Khomeini.

In Tabriz, meanwhile, capital of East Azerbaijan province in the northwest, militiamen loyal to the Ayatollah Kazem Shariatmadari released nine Revolutionary Guardsmen taken hostage in a clash last Thursday. The release followed an appeal to the militiamen by Shariatmadari.

After his meeting with Ghotbzadeh, Waldheim withdrew to his hotel room for the rest of the day, receiving European ambassadors to Iran, awaiting an itinerary for his visit from the Foreign Ministry and avoiding the press.

An afternoon trip in which the U.N. envoy was scheduled to visit Iranians crippled in fighting during last year's Islamic revolution was canceled after a hostile crowd gathered outside the building he was to visit, the old officers club of the deposed shah's secret police, SAVAK.

The crowd chanted anti-Waldheim slogans and waved photographs of him kissing the hand of Princess Ashraf Pahlavi, the shah's twin sister.

Hours after the trip was put off, Ghotbzadeh announced that a plot against Waldheim's life had been discovered and thwarted this afternoon. But the Foreign Minister offered no evidence and his statement was seen as an excuse to keep Waldheim's movements secret so as to avoid further demonstrations such as today's which formed at the broadcast urging of the students at the U.S. Embassy.

Chanting "America, America, death to your dirty tricks," the demonstrators blocked the entrance to the building. Protesters said they viewed Waldheim as an American puppet who came here to deceive the students into freeing the American hostages.

Khomeini, Iran's Islamic ruler and the only man who appears to have any real influence over the radical students, still had no plans today for meeting Waldheim. His associates on the ruling Revolutionary Council all but ruled out a Waldheim session with the 79-year-old iman.

The Islamic Republic newspaper, which consistently reflects Khomeini's views, published a photograph in today's editions of Waldheim shaking the shah's hand -- an association that denigrates the U.N. envoy here. The newspaper has criticized Waldheim for ignoring the abuses of the shah's regime in the past.

It is believed that Khomeini may be too preoccupied with the brewing problems in Iran's provinces -- which present him with a more serious challenge to his rule than the U.S.-proposed threat of economic sanctions -- to focus on Waldheim's mediation attempt.

Some diplomatic observers believe Khomeini views the hostage issue as a possible rallying point to unite residents of the querulous provinces, distracting them from their fight for autonomy and providing Khomeini some room for maneuvering.

Waldheim is known for his patient, methodical style of diplomacy and the 61-year-old envoy seems to have set for himself the initial task of breaking down Iranian distrust of the United Nations by trying to convince them that the international body is impartial and sympathetic to their past sufferings.

Only then can Iranian officials be expected to accept a proposal to vent their grievances before an international panel set up under U.N. auspices as a condition for the release of the American hostages who have been held by militant Islamic students in the U.S. Embassy here for 60 days.

Although the same plan was rejected here out of hand several weeks ago, Waldheim is understood to feel it could work today because of the growing sense of isolation, among Iranian leaders and the lack of other alternatives to resolve the crisis that is distracting officials from pressing domestic problems.

In addition, Iran is said to have come under increasing pressure from other Islamic nations to end the hostage problem since the Soviet military intervention in neighboring Afghanistan, which is seen as a threat to other Moslem countries in the area.

Despite the critical Iranian news articles and the unfriendly demonstrations that marred Waldheim's first full day here, there were some official signs of interest in his mission and indications that the government may be trying to soften public opinion on his mediation efforts.

In a report by the Foreign Ministry of the Waldheim-Ghotbzadeh talks this morning, the secretary general was described as showing great concern for human rights, especially Iranian suffering under previous regimes. No direct mention was made of the hostages. Throughout the hostage crisis, Iranian officials have expressed frustration that the U.S. is only interested in the release of its embassy captives and shows no concern for their grievances under the 25-year rule of the shah, who was supported by the United States.

In announcing a death threat to Waldheim, Ghotbzadeh said that the potential danger to the secretary general was serious enough to keep his future travels private. It is believed that the foreign minister may have used the alleged plot to prevent future antagonistic protests by Iranians.

In another possibly positive sign, Ayatollah Mohammed Behesti, head of the Revolutionary Council, said this morning that the governing body would have "a general meeting" with Waldheim or council members would meet with him in separate groups.

Behesti, a respected political figure in Iran who represents moderates within the Council, said Waldheim's mission here "will help to find a wise solution to the problem."