It came as no surprise to U.S. government experts on Iran that a seemingly obscure Islamic clergyman, Mohammed Mousavi Kho'ini, was named yesterday by former hostages as the "mentor" of the militant students who seized the American Embassy in Tehran. That is what the government analysts have believed almost from the start.

Kho'ini, who is in his early 40s and is considered a protege of Ayatollah Ruhollah Khomeini, appeared at the embassy compound shortly after it was taken over by Moslem students on Nov. 4, 1979, and began convening daily meetings and leading daily prayers. Later, Kho'ini was elected to the parliament, or Majlis, and served as chairman of the legislative commission that formally established the terms for release of the Americans.

In an interview with a Tehran newspaper last July, Kho'ini said that before the takeover of the embassy he had been approached by five to seven students who "told me about their intentions and also sought my opinion to know whether such an act was in line with Imam Khomeini's viewpoints." Kho'ini went on to say that he declined to broach the matter to his senior leader lest this put the ayatollah in a difficult position.

"Following my opinion," Kho'ini continued in the interview with the newspaper Jomhouri Islami, "the students decided to seize the embassy and to keep the matter a secret until the appointed day. It was also agreed that I should join them from the very outset of this seizure." Elsewhere, the students who seized the embassy said that planning had taken about 10 days. t

The cleric's account was consistent with the statement at the news conference yesterday by Victor Tomseth, chief of the political section of the U.S. Embassy, who named Kho'ini as "mentor" of the students who seized the compound. Tomseth said he did not believe the then-existing government of Medhi Bazargan, which was brought down by the embassy takeover, had much advance knowledge.

Tomseth was held in the Iranian Foreign Ministry during most of the past 14 1/2 months, but John Graves, who was held inside the embassy compound, added in yesterday's news conference that there is "enormous . . . truly cogent evidence" that the captors were "legitimate students." Graves appeared to be reacting to news reports that the captors were expert terrorists associated with the Palestine Liberation Organization or other international bodies. State Department officials said yesterday they have no evidence of this, although some persons who had received training from a branch of the PLO were later reported to be in the compound.

In the interview last July with Jomhouri Islami, a Tehran daily, Kho'ini said the embassy takeover "was initially motivated by the American support for the ousted shah." He did not indicate that the actual return of the late shah, Mohammad Reza Pahlavi, had been an initial objective, though this quickly became one of the expressed demands of the captors.

The cleric also mentioned in the same interview two other initial aims of the seizure. One was to "overshadow" deviations appearing in the Iranian revolution. He also said that the students were reacting to information that "a very important CIA official" had entered Iran and was staying at the embassy. This was never confirmed.

The repercussions of the capture of the Americans, which Kho'ini and other Iranians later described as historic in scope, apparently were not foreseen by those who carried it out. "Before the embassy seizure, we didn't anticipate this issue to take such dimensions," Kho'ini said in the interview. a

Kho'ini, who is sometimes erroneously referred to as an ayatollah, has the clerical rank of hojatolislam, which is a senior preacher. According to a brief biography given by him to a western reporter, Kho'ini began his theological education in 1956 as a young student, and by 1972 became preacher of a Tehran mosque next to the palace of the shah.

Kho'ini said he was imprisoned in 1976-77, placed in solitary confinement for seven months and tortured by SAVAK, the shah's secret police. After his release he traveled to Paris to aid Khomeini, his mentor, staying with him there until the ayatollah returned to Iran in triumph on Feb. 1, 1979, following the fall of the shah.