Radicals Hold Sway in Tehran. SEARCH FOR KHOMEINI SUCCESSOR

IRAN'S Council of Religious Experts will soon meet to choose a new successor-designate to Ayatollah Ruhollah Khomeini, the country's supreme leader, according to a source close to Mir Hossein Musavi, Iran's prime minister. The 70-member group may set up a council of three or five religious leaders to take over when Ayatollah Khomeini dies, the source added.

In 1985, when the council last met, it chose Ayatollah Hossein Ali Montazeri to be Khomeini's eventual successor - knowing that Khomeini personally preferred Ayatollah Montazeri.

However, Montazeri resigned Tuesday over disagreements with Khomeini, becoming the highest-ranking victim of a widening shake-up by radicals in Tehran. Deputy Minister of Foreign Affairs Muhammad Javad Larijani and Iran's representative to the United Nations, Jaafar Mahallati, also resigned recently.

Iranian officials contacted in Tehran after Montazeri's resignation insisted that the situation throughout the country was calm.

``Montazeri's departure doesn't make any problem,'' the source near the prime minister said.

Iranian exiles contacted in Paris and London were not that positive. They said Montazeri's resignation may lead to mass arrests of his supporters and of liberal and moderate Iranian politicians who were until now under his protection. Among those is former Prime Minister Mehdi Bazargan, for years an open critic of the government.

The exiles also said Montazeri's withdrawal may well be the beginning of the disintegration of Iran's Islamic regime.

``There is presently no senior cleric who has the prestige to be a successor or to sit on a succession committee,'' said a source in Paris who is close to former president Abolhassan Bani-Sadr.

The conflict between Khomeini and his appointed successor became apparent on Feb. 6. In a newspaper interview Montazeri said, ``The goals of our revolution have not been achieved because of political incompetence and factional fighting.''

Six days later, on the 10th anniversary of the 1979 revolution, Montazeri said, ``Often we showed obstinacy, shouting slogans that shut us off from the rest of the world. The people of the world thought our only task in Iran was to kill people.'' MONTAZERI had also questioned the wisdom of Iran's political intransigence during the eight years of war with Iraq. He had suggested that Iranian leaders adopt a more moderate stance on the international scene in order to break the country's diplomatic isolation.

And Montazeri had insisted that moderate and liberal dissidents be allowed to take part in Iran's political life.

The crisis culminated on March 21, the start of the Iranian new year. The Voice of America broadcast the contents of two confidential letters written last July by Montazeri to Khomeini. In one, he accused the guide of the revolution of having personally ordered the killing of hundreds of jailed opponents.

The Iranian rebel group, the People's Mojahedin, which leads the armed struggle against the Islamic regime, says that 12,000 of Tehran's opponents have been executed since July last year. In a report published on Dec. 13 Amnesty International said at least 300 dissidents were shot between July and December 1988.

``This genocide is incompatible with Islam,'' Montazeri reportedly wrote to Khomeini. The next day, Khomeini ordered Montazeri to publicly recant. Montazeri refused.

The main beneficiary of the present confrontation is the Combatant Clergymen Society, according to an Iranian just back from Tehran. The society is a radical group led by the Deputy Speaker of the Majlis, Mehdi Karrubi.

``The society now enjoys a majority within the parliament which allows its members to impose their will on Iran's political elite. They also have the support of Ayatollah Khomeini,'' the Iranian explained.

Among this group is Interior Minister Ali Akbar Mohtashemi, who for months has openly ignored calls by Montazeri to allow some form of political dissent to be expressed in public.

Mr. Mohtashemi, a former ambassador to Syria, is accused by Western intelligence services of maintaining links with groups holding Western hostages in Lebanon.

Another member of this new ruling elite is Musavi Khoeiniha, once the spiritual leader of the students occupying the United States Embassy in Tehran.