Massoud Rajavi, the leader of Iran's most prominent leftist guerrilla organization, could emerge to face the country's Islamic rulers with possibly the most potent challenge since the 1979 revolution, according to European diplomats and analysts specializing in Iran.

Rajavi, little-known outside of Iran, is the leader of the Mujaheddin-e-Khalq, an Islamic, Marxist underground organization and, as such, a powerful new figure for Iran's clerics in the Islamic Republican Party to reckon with, according to these sources.

The Mujaheddin stepped to center stage in Iran's politicl power struggle after the country's most powerful figure, Ayatollah Ruhollah Khomeini, blamed them for carrying out the recent Tehran bombing that killed the second-ranking official in the Islamic hierarchy, Ayatollah Mohammed Beheshti, and 71 other leading members of the Islamic Republican Party. Although no group claimed responsibility for the bombing, observers in Europe generally believe the Mujaheddin were involved.

The group, officially banned during the reign of the late shah, Mohammad Reza Pahlavi, first moved above ground in the lat 1970s with the start of the revolution that deposed him. It late formed an alliance with then-prsident Abol Hassan Bani-Sadr. The London-based magazine Eight Days said in an article published last week that Bani-Sadr was being sheltered in a remote area of Kurdistan in the company of Mujaheddin bodyguards.

The bomb attack last month is seen as a sign that Bani-Sadr's fall has forced the Mujaheddin to return to their underground roots and resume violent tactics. A prominent Iranian exile in Britain, who did not want his name used, said that the Mujaheddin were the only ones likely to have the materials, expertise and intelligence network needed for the job.

"Now that they have returned underground," he added, "they are even more dangerous to the [Islamic Republican Party]. . . . They have a well-organized system and extensive contacts. The Mujaheddin now pose a major threat to the regime."

Rajavi, 33, like may of the Mujaheddin now under his command, was recruited from Tehran University where he studied law and political science in the late 1960s and early 1970s.

In 1974, Rajavi and several of his Mujaheddin compatriots were arrested by SAVAK -- the shah's secrect police -- after a series of guerrilla attacks. All except Rajavi were executed. He was sentenced to life imprisonment.Why he was spared is uncertain.

Rajavi is believed to have played a role in guiding Mujaheddin attacks from his prison cell on American targets, which led to the slaying of three U.S. Air Force colonels between 1973 and 1975, a Rockwell International technician in 1976 and an American oil executive in 1979.

One Iranian exile who has visited Iran since the revolution and met Rajavi, said the guerrilla leader has watered down some of his Marxist ideals and tailored them more closely to the Iranian experience. He has, for instance, become an even more devout Moslem and insisted on Iran's independence from foreign organizations.

The Mujaheddin now appear to have stolen the thunder from the other two major leftist organizations in Iran. The pro-Moscow Tudeh Party has hitched itself to Khomeini and the Islamic Republican Party, but diplomatic sources believe that its aging leadership is too closely associated with Moscow and devoid of new ideas to win any widespread popular support. The Fedayan, a pure Marxist-Leninist guerrilla organization, has failed to win support from the masses, these sources said, because of its rejection of Islam.

The Mujaheddin's appeal is that is claims to be Islamic as well as Marxist, although it remains unclear how these two apparently contradictory sets of values can coexist. Since the downfall of the shah they have recruited widely among those looking for radical change but who are unwilling to give up their religion. Analysts believe the group has become so successful in recruiting that it may become the umbrella organization for leftist groups in Iran.

Both the Mujaheddin and the Fedayan played a major role in the street fighting in 1978 that marked the final days of the shah's rule. Both clashed with Khomeini's Revolutionary Guards in attempts to occupy the Pahlavi palace. The Mujaheddin and Fedayan enjoyed a brief spell back in the clerics' favor after the occupation of the U.S. Embassy when Iranians were terrified of the possibility of U.S. military retaliation and anyone with guerrilla training was in demand.

The Mujaheddin tried to capitalize on the embassy drama.Some Mujaheddin guerrillas wee among the first into the embassy compound, but they were soon expelled by the Revolutionary Guards. Diplomatic sources believe that the Mujaheddin, rejected by Khomeini, were then faced with the choice of either returning underground or allying themselves with then newly elected Bani-Sadr.

Bani-Sadr was, according to these diplomats, looking for allies to make up for the absence of a party structure of his own and already under attack from the Islamic Republican Party. The Bani-Sadr-Mujaheddin alliance was shaky from the start, united almost solely by common opposition to the ruling clerics.

As soon as Bani-Sadr was dismissed, the ruling party and Revolutionary Guards shifted the focus of their hate almost entirely to the Mujaheddin, sources said. It was reported that within 10 days up to 2,000 persons were arrested --most of them Mujaheddin members or supporters. Since then a number of them have been executed.

Experts on Iran say that as long as Bani-Sadr ws in power the Mujaheddin were prepared to pursue their objectives through the postrevolution political process. Now that he has been dismised, the only channel open to them is violence. One well-informed source said, "They used these tactics effectively to bring down the well-organized regime of the shah. With the chaos that now exists in Iran, their task is much easier."