To his supporters, he is Ayatollah Ruhollah Khomeini's strong right hand, a man of God working to steer the country toward an Islamic cultural revolution. His detractors call him "Rasputin" and accuse him of being a political opportunist in clerical garb.

Few, however, would dispute that Ayatollah Seyed Mohammed Hossein Beheshti, 51, a member of the ruling Revolutionary Council, is one of the most powerful men in the new Iran and a key figure in determining its future. More than anyone else, he wields the power behind the throne. His authority probably ranks second only to Khomeini's, overshadowing that of his chief secular rival, President Abol Hassan Bani-Sadr.

Beheshti is considered the most ambitious proponent of having the clergy actually run the government, instead of spiritually "guiding" the revolution from the mosque as Khomeini advocated before returning to Iran from exile last year.

Aside from the political power that is associated with him Beheshti also symbolizes the new economic force that Iran's Islamic clergy has become since the February 1979 revolution. According to a document obtained by The Washington Post, Beheshti sits on the board of directors of a private company that did business under the regime of deposed shah Mohammad Reza Pahlavi.

Iranian dissidents believe the ayatollah has other financial interests, notably large land holdings, and is a wealthy man in his own right. Asked in the past about his property by Iranian interviewers, Beheshti has divulged little. He usually has responded only by bursting into laughter in his resonant, baritone voice.

Indeed, maintaining a certain mystery also seems essential to the way Beheshti operates politically. According to diplomatic surces, he met before and after the revolution with American officials, including the former ambassador to Iran, William Sullivan. Beheshti has never publicly acknowledged these meetings.

At the climax of the revolution in February 1979, Sullivan asked Beheshti to rescue about 20 Americans besieged in building in an Iranian military compound by revolutionaries. Beheshti then arranged a cease-fire and a convoy to take them to the U.S. Embassy in the middle of the night.

Despite his current reputation as a right-wing clerical hardliner, diplomats have generally considered Beheshti a pragmatist with a sharp sense of political expediency, a man with whom Westerners could deal and seek an accommodation. He speaks passable English and German, having directed a mosque in Hamburg for five years.

While he keeps his political maneuverings and business dealings under wraps, Beheshti, whose name means "heavenly" in Persian, maintains a certain public profile by holding regular press conferences that foreign correspondents may attend.

He cuts an imposing figure, but his occasional witticisms belie the austerity normally associated with his black turban and clergyman's robes. At times, especially when he laughs, and reveals a missing front tooth, his long salt-and-pepper beard and rugged, roguish face make him look like a pirate.

Although Beheshti's carefully worded but usually vague pronouncements leave his politics open to interpretation, some diplomatic analysts in Tehran feel he would be most comfortable with a one-party state under his control -- what one calls a kind of "Islamic Stalinism."

Many Iranians reproach him for his deviousness in pursuing just such power, an image to which Beheshti himself has generously contributed. In an oft-cited speech to high school students, he once said: "It is not enough to be honest and simple; one must be honest and cunning."

While most of Beheshti's power is exercised behind the scenes, through proteges and intermediaries, his positions show the extent of his influence.

Not only is he a leading member of the Revolutionary Council, but Khomeini has appointed him chief of the Supreme Court, and he is a founder and spiritual chief of the Islamic Republican Party, the country's largest and best organized.

Beheshti headed the Revolutionary Council until Bani-Sadr's presidential election victory in January, and still effectively controls the body. Even if he may not always command a majority in the council, he can depend on enough support to scuttle any major decision.In practice such decisions must be adopted unanimously before they can be enforced.

It was Beheshti's behind-the-scenes opposition, for example, that helped destroy Bani-Sadr's efforts earlier this year to transfer the American hostages to government control.

As chief of the Islamic Republican Party, Beheshti effectively controls Iran's new parliament. At least 130 of its 270 seats have been won by the party's candidates.

His influence over the party also gives Beheshti effective command over the streets of Tehran and the mobs of hezbollahi, or "followers of the party of God," that have been used to crush leftist or liberal political opposition to clerical power.

In addition, Beheshti is believed to wield strong influence in the Revolutionary Guards and the komitehs, which respectively function as an army and police force loyal to Khomeini and more or less independent of the secular government.

More than anyone else in revolutionary Iran, therefore, Beheshti directly exercises power in the executive, legislative and judicial branches of the government, in the security forces and over the Tehran masses.

"He controls all the vital organs of the country," said a dissident Iranian politician. A lot of people are talking about a Beheshti dictatorship. He's really a power-hungry politician in clerical robes."

Even Fbeheshti's critics, though, credit him with being a shrewd tactician and an able manager. Diplomats generally consider him the keenest political mind in the country.

Much less is known about Beheshti's economic interests. He is widely reputed to have acquired a considerable fortune in real estate, but details have remained mysterious.

One item that has surfaced, however, is Beheshti's involvement in a firm called Ahdaf, which is registered, as required by law, in Iran's Official Gazette as a "commercial and construction company."

In an announcement last fall of a change in the firm's management, Beheshti was listed as a member of Ahdaf's five-man board along with two close associates, Revolutionary Council member Mohammed Javad Bahonar and Education Minister Mohammed Rajai.

Asked in a brief interview about his involvement with the firm, Beheshti said it was "only an institution which was responsible for ownership of religious private schools . . . and not a benefit institution," meaning a profit-making company.

When it was pointed out that the Official Gazette listed Ahdaf as a "commercial and construction company" and noted that its statement of profit and loss for the previous year had been approved, Beheshti explained, "At the time of the last regime if we registered this institution as nonbenefit, the government would lay its hands on it. To avoid this we have registered it as a benefit institution. But there is really not any benefit. The money is not our money, but belongs to the people."

Iranians familiar with the country's company law and procedures say that a clerical role in such a firm is unusual and the Beheshti's explanation does not make sense.

Many institutions were registered as nonprofit under the previous government, and it was not even necessary to form a company for the purpose he described, these sources say.

Contributing to the controversy surrounding Beheshti have been allegations that he tacitly collaborated with the shah's regime before the revolution, notably during his tenure as director of the Hamburg mosque.

Some Iranian students who knew him then have charged that he avoided criticizing the shah and made sure that an Iranian student newspaper he supervised steered clear of such controversy.