Tens of thousands of Iranians, joined for the first time by a Marxist guerilla organization, today staged the biggest demonstration so far in support of the radical Islamic militants holding about 70 hostages at the American Embassy.

Earlier today at a rally at Tehran University, a student leader warned that if the United States allowed the deposed shah to move to another country, the hostages would "find themselves in more difficult conditions." He did not elaborate.

There appeared to be no progress in U.S. efforts to win release of the hostages.

Meanwhile, there was confusion and disagreement between Iranian officials about whether Iran has decided to stop accepting U.S. dollars for oil payments.

Members of the Marxist Fedaye Khalq guerrilla movement, making their first public appearance in months, joined in today's demonstration at the U.S. Embassy.

They had gone underground during the summer, apparently fearing an armed showdown with their more numerous rivals in the rightist Islamic movement, which enjoys official support. Despite fears of a clash between left and right today, the demonstrations took place without reported incidents. l

Large crowds milled around the front gates f the embassy in a carnival mood, with vendors selling food, ice cream, posters and drinks.

Despite hopes raised by members of the ruling Revolutionary Council yesterday that non-whites among the hostages might be released soon, student spokesmen at the embassy today stuck to their hardline refusal.

"No one has the right to talk about the release of hostages on our behalf," a student spokesman said, "except the imam," as Iranians refer to Ayatollah Ruhollah Khomeini, the country's religious ruler.

The spokesman said the State Department had telephoned from Washington to inquire whether "black and yellow hostages" had been released. Several of the dozen non-American hostages are Asians.

The student leader at Tehran University, apparently referring to reports that Shah Mohammad Reza Pahlavi might return to Mexico after completing cancer treatment in New York said:

"It seems the United States wants to move the shah. That would be worse. If they send him away, the hostages will find themselves in more difficult conditions."

The students are demanding that the United States return the shah to Iran for trial on charges of murder and other crimes allegedly committed before he was deposed a year ago.

At the Tehran University meeting, Ayatollah Hossein Ali Montazeri criticized oil producing countries for "refusing to answer our appeals for help" in cutting off their own oil supplies to the United States.

"It's enough that they cut oil to the United States for one or two days to realize the aspirations of the disinherited," the ayatollah said, before warning, "The Arab leaders should remember the fate of the shah."

Meanwhile, Ali Akbar Moinfar, the head of the National Iranian Oil Co. and the country's de facto oil minister, said he had 'neither received nor given orders to refuse dollars for payment" for oil. Iran, he said, "will proceed as in the past in respect to payments."

Abol Hassan Bani-Sadr, who holds the foreign, finance and economy portfolios, said yesterday, and repeated today, that he had ordered the dollar ban for oil payments Tuesday after President Carter ordered a U.S. boycott of Iranian oil.

"I am the finance minister, and I decide," Bani-Sadr said tonight. There was no immediate resolution of the differing statements.

Suggestions of a change from the dollar to other currencies have been made by some oil producing countries in the past, largely because of the diminishing value of the dollar, but no action has been taken elsewhere.

Financial experts said yesterday that a switch by Iran from the dollar to other currencies would have little effect on the dollar, but that if other countries joined Iran, the effect could be a run on the dollr that would further undercut its value.