Moslem radicals, led in part by Iranian supporters of the Ayatollah Ruhollah Khomeini, have seized control of Friday prayer services at Washington's Islamic Center in the last three weeks.

The power struggle, which marks the first significant radical Moslem activity in the Washington area since the Iranian student demonstrations here in July and August, was triggered by the radicals' conviction that the American Moslem community has failed to rally behind the Iranians in their war with Iraq, sources said.

The challenge is apparengly being directed by a group called the Majlis Shura, whose secret membership reportedly includes Bahram Nahidian, the Iranian-American rug dealer in Georgetown believed by law enforcement officials to be the leader of pro-Khomeini activity in the United States.

The radicals have refused to allow the Islamic Center's acting director, Muzammil Saddiqi, who is also the imam, or religious leader, for the center, to lead the Friday prayer services. Instead, some of the radicals have led the service, center officials say.

At the same time, at least one Islamic Center staff member claims he has received three death threats in the last week from individuals claiming they support Iran. Yesterday, the center received an anonymous phone call from an individual who warned that supporters of Iran will seize total control of the mosque today. The D.C. police department's special operations division has been alerted and will be on hand for the services.

The sudden reemergence of the radicals comes at the same time as a federal grand jury here is investigating possible violations of law by a group called the Islamic Guerrilla Army (IGA). Law enforcement officials say they suspect the group may have among its members terrorists and they have named Nahidian as the head of the group.

Several alleged members of this group have appeared before the grand jury, which was convened in the U.S. District Court several weeks ago.

Law enforcement sources say the scope of the grand jury probe is broad, with prosecutors attempting to discover more about the actions of the IGA.

A task force of about two dozen FBI agents working out of national headquarters here has been interviewing scores of Iranians in the last few months to determine if there is a conspiracy to violate federal laws.

Law enforcement officials say they believe the IGA comprises about 50 persons -- Iranian supporters of Khomeini and radical American Moslems -- who are heavily armed and determined to suppress Khomeini foes with force.

It is this group, law enforcement officials say they believe, that was responsible for the assassination last July of Ali Akbar Tabatabai, a leader of anti-Khomeini forces here who was gunned down in his Bethesda home.

The FBI has charged Daoud Salahudden, also known as David Belfield, with the murder. He is now believed to be in Tehran. Law enforcement officials believe Salahuddin was a member of the IGA and that he was a close associate of Nahidian's.

Members of the IGA, and pro-Khomeini demonstrators in general, have been subdued in recent months after staging numerous demonstrations in July and August. Law enforcement officials attribute that to the intensity of the official probe into their activities. "They are keeping a low profile right now," said one official familiar with the pro-Khomeini group here. Some officials speculate that another reason for the group's lack of activity is that the militants are not sure who among them may be providing information to the FBI.

Law enforcement officials believe the Majlis Shura is the ruling committee of the IGA. "They're supervisors, a little committee of seven to eight people -- it's the upper structure of the IGA," one official said.

The normally placid mosque, located at 2551 Massachusetts Ave. NW, and technically the spiritual center for the Washington area's 50,000 Moslems, has been in turmoil since last spring when many of the same radicals, including Nahidian, began pressing for the dismissal of Mohammad Abdul-Rauf, director of the center.

Eventually, Saddiqi was named by the center's board of directors to replace Rauf, who will leave officially next month. Saddiqi stated at the time that he felt religion and politics could be discussed during prayer services, a key demand of the radicals who were eager to recruit other Moslems to their cause.

But a senior Islamic Center Official said yesterday that almost from the beginning it was apparent "that they [the radicals] would never settle for anything but total control of the mosque."

Nahidian refused to comment. "I will not talk about Islam with anyone who is not Moslem," said Nahidian, who gave last Friday's sermon in the mosque and was among those who refused to allow Saddiqi access to the pulpit. "You are an enemy of Islam."