The Carter administration, which had pinned its hopes for rescuing the hostages at the U.S. embassy in Tehran on former attorney general Ramsey Clark's mission, found itself last night with the mission's future in doubt and fears for the safety of the prisoners becoming more pronounced.

At the same time that the State Department was announcing the delay of the Clark mission, U.S. sources said they had received information from Tehran that the 60 to 65 Americans being held prisoner were being psychologically and verbally abused by their captors.

However, the sources stressed, they did not have any hard evidence that the hostages had been subjected to injury or severe physical mistreatment beyond the harassment stage.

These new concerns climaxed a day that began when President Carter raised hopes for the safe release of the hostages by announcing that Clark and William G. Miller, a staffer of the Senate Intelligence Committee, were en route to Tehran to discuss the situation with Iranian government and religious authorities.

Carter, making clear that he considered Clark's trip essential to a bloodless resolution of the crisis, replied to reporters' questions about the move's chances of succeeding by saying: "We hope it will. We pray it will."

But, as the day wore on, the initial statements by U.S. officials about having assurances that Clark and Miller would be received were overtaken by ominous reports coming out of Iran. They quoted the Ayatollah Ruhollah Khomeini, the Moslem religious leader controlling Iran, as saying his country's religious and secular officials would not meet with the U.S. emissaries.

That led U.S. officials into a frantic scurrying for clarification and, late in the afternoon, State Department Spokesman Hodding Carter appeared before reporters to read a statement that said:

"As the spokesman said today, the Iranian authorities had agreed to receive the emissaries. However, we were informed this afternoon by authorities in Iran that Mr. Clark and Mr. Miller should not proceed to Tehran at this time.

"They will remain in Istanbul pending clarification of the position of the government of Iran. It is our hope that they will be able to proceed with their mission at the earliest opportunity."

Administration officials, elaborating privately on the statement, said they could not tell immediately what had happened to change what earlier had been considered a firm commitment by the Iranians to receive Clark.

As a result, there was no way of saying last night, the officials stressed, whether the Clark mission could be put back on track or whether it would be aborted with Clark and Miller never getting beyond Istanbul.

U.S. sources said that, since the takeover of the embassy Sunday, the administration's principal emphasis has been on finding some way of breaking through the political chaos in Iran to appeal directly to Khomeini or those close to him for the hostages' safety.

From the outset, the sources continued, the idea of sending a special envoy to Tehran was one of the main approaches under consideration, and Clark was contacted Monday to take on the task. He was chosen, the sources said, because he is a prominent American public figure who had been outspokenly critical of the shah's rule and who had talked extensively with Khomeini in France last year, before the shah's overthrow.

Miller is a former Foreign Service officer who had extensive experience in Iran during the 1960s and who is acquainted with some of the people now playing roles in Khomeini's islamic revolutionary movement.

While efforts were underway to get permission for them to visit Iran, the sources said, a number of other avenues were pursued, including efforts to enlist other governments to intercede with the Iranians.

But the sources noted, although almost every government contacted expressed willingness to help, they added that their influece in Iran was so minimal that there was little they could do.

As a result, the source said, increasing stress was put on trying to get the Iranians to receive Clark -- a task that was complicated greatly when the government of Prime Minister Mehdi ybazargan resigned Tuesday. That forced the United States to deal with relatively low-ranking career bureaucrats in the Iranian foreign ministry.

Despite the difficulties, plans to send Clark and Miller moved ahead, with the final review of their instructions taking place at a National Security Council meeting Tuesday night. But, U.S. officials said, it wasn't until 1 a.m. yesterday that what was considered authoritative permission for them to make the trip was received from Tehran.

Although they would not say specifically who had given the permission, the officials said it "went beyond the permanent government" -- an indication that it came from the Khomeini-controlled Islamic Revolutionary Council.

The later reversal of this permission, which U.S. officials said was relayed to Washington both indirectly through unspecified offical channels, appeared to indicate that some kind of factional split between Khomeini and other council members might be involved.

Clark and Miller, who conferred both with Carter and Secretary of State Cyrus R. Vance before leaving Washington, were flown by U.S. government planes first to Athens and then to Istanbul. Before they were ordered to mark time in neighboring Turkey, they had been scheduled to arrive in Tehran last night on a commercial airline flight.

In a related development yesterday, Rep. Paul Findley (R-Ill.) announced that he had telephoned the Palestine Liberation Organization in Beirut on Tuesday and had urged PLO leader Yasser Arafat to help rescue the Americans. A two-man PLO team led by Arafat's military commander, Abu Jihad, since has gone to Tehran.

Findley said he had talked to State Department officials on Tuesday to urge that the administration ask Arafat for help but had been unable to get a reversal of the U.S. policy of refusing to deal with the PLO.Asked about the PLO's intercession yesterday, Hodding Carter said, "We would welcome help from anyone." CAPTION:

Picture 1, Ramsley Clark. . .mission temporarily halted; Picture 2, Sens. Jacob Javits, and Frank Church after closed Hill on Iran. UPI