The son of Ayatollah Ruhollah Khomeini today labeled fanatical Moslems "traitors" to Islam for trying to break up a political rally held Thursday by a leftist political party.

With a strongly worded statement to the Pars News Agency, Hojatoleslam Ahmad Khomeini placed himself in direct opposition to the fundamentalist Islamic Republican Party that controls the newly elected parliament. The party's newspaper, Islamic Republic, today accused the left-wing Mujaheddin-e-Khaiq of provoking the clashes by holding the political rally, at which at least one person was killed and about 300 injured.

It is nuclear whether Ahmad Khomeini, 35, the appointment secretary and gatekeeper for his ailing, 80-year-old father, was speaking for Ayatollah Khomeini.

Meanwhile, one of the students controlling the U.S. Embassy here said today he had been beaten up at the Thursday rally in the soceer stadium across the street from the embassy compound. The stadium was to have been used as a helicopter landing point in the abortive attempt April 25 to rescue 53 U.S. hostages held since the embassy was seized Nov. 4.

In his statement today, Ahmad Khomeini said he was so upset by the attack on the Mujaheddin's ray by Moslem fundamentalists called Hezbollahi that he broke his self-imposed silence to speak out on the issue.

"Dear imam has said there is freedom of expression," Khomeini said, referring to his father. The officials say there is freedom of expression. Then why do such things happen?"

He said the fundamentalist attackers, who call themselves followers of the "party of God" and are recruited from the lower-class districts of teaming south Tehran, had discredited Islam. The group's leaders should be arrested "so that Islam is cleansed from the filthy existence of those traitors," Ahmad Khomeini said.

"If the slogan, "There is no party but the party of Allah,' means beating up," Khomeini said "then they have discredited Allah."

Ahmad Khomeini, a quiet, self-effacing man, has been a strong supporter of President Abol Hassan Bani-Sadr and has been one of the few voices of moderation among the ruling clergy who control Iran. But it is not known how much sway he holds over his father or how much he reflects the ayatollah's views.

Ayatollah Khomeini warned Tuesday that a lack of unity among this country's leaders threatened the Islamic revolution.

While he was not specific in his Tuesday talk, there has been an increasing amont of public friction between Bani-Sadr and Ayatollah Mohammed Beheshti, leader of the Islamic Republican Party. Most of this has been carried in their respective newspapers.

Beheshti, for example, has apposed Bani-Sadr's right to pick a prime minister, favored the closing of universities to eliminate Western, non-Islamic influence and thwarted Bani-Sadr's attempts to bring the American hostages under government control. The Islamic Republican Party Thursday accused Bani-Sadr's newspaper, Islamic Revolution, of violating "Islamic morality and character" by explaining that Beheshti was trying to undercut the president when Bani-Sadr named his candidate for prime minister. u

Today, Beheshti's Islamic Republic newspaper accused the Mujaheddin of trying to cause clashes by holding rallies at a time when Iran is faced with external threats. The paper accused the leaders of the leftist parties of being "opportunistic, adventuristic and inhumane."

For his part, Bani-Sadr came out with an editorial 10 days ago in Islamic Revolution calling for an end to political violence in an obvious reference to the Hezbollahi, who are generally considered to be supporters of the Islamic Republican Party.