Jubilant smiles turned swiftly grim here tonight as a last-minute snag held up release of the 52 U.S. hostages whose freedom was worked out in a complicated accord between the United States and Iran initialed by chief U.S. negotiator Warren Christopher.

[Early Tuesday, there were reports in Washington that the unexpected hitch had been resolved and that the Carter administration had ordered banks to begin moving frozen Iranian assets into the Federal Reserve -- the first step toward release of the hostages. Although U.S. officials said that President Carter had called Christopher to tell him final agreement had been reached, there was no word from Algiers on what course of action Christopher had been instructed to take.]

U.S. Amabassador Ulrich Haynes, who had been relaxed and smiling around the embassy compound here during much of the day, rushed off to the Algerian Foreign Ministry with two U.S. financial experts and returned alone looking concerned. An Algerian central banker involved in the protracted negotiations that seemed this morning to have been successfully terminated canceled an appointment this evening with a british financial journalist.

The delay appeared to revolve around implementation of the U.S. side of the bargain, in which Washington agreed to "restore the financial position of Iran insofar as possible" by unfreezing Iran's assets blocked in November 1979 by President Carter in retaliation for the hostage-taking.

"Both sides have signed the agreement, but they are still sorting out details," said an embassy spokesman.

Christopher, reflecting the upbeat mood and wide assumption that the marathon negotiations were over, had changed into casual sportswear after the formal signing ceremony this morning in the Treaty Room of the Algerian Foreign Ministry.

But he changed back into business clothes for an ministry this evening for a 3 1/2-hour meeting -- one of his longest uninterrupted sessions there during the final negotiations he has headed here since his arrival in Algiers on Jan. 8. He returned later in the evening for more talks.

"At last I can smile," was the comment of the usually taciturn Christopher early in the day, just before the 10-minute initialing ceremony at 8:30 a.m. Algiers time [3:30 a.m. EST].

Even before the ceremony Algiers radio had let out the news that two Algerian Boeing 727 airliners had flown to Ankara, Turkey, on the way to pick up the hostages and bring them here for a first stop on the way to freedom.

It was expected that U.S. Air Force medical evacuation planes would immediately pick up the 50 men and two women in Algiers for transfer to the U.S. Air Force hospital in Wiesbaden, West Germany, for a number of days of medical and psychological examinations.

The approximately 150 reporters and cameramen in Algiers were issued special passes for access to the restricted areas of the Algiers airport, where the freed hostages were expected to land after leaving Iran. An Algerian Foreign Ministry spokesman said that news representatives only would be allowed to film the freed diplomats from a distance.

Obviously thinking that he was setting the final seal on the accord this morning, Algerian Foreign Minister Mohammed Benyahia, who has been Christopher's partner in the negotiations, said:

"We've spent the past 48 hours without sleep to arrive at this happy outcome."

The Algerian then invited Christopher the initial the three Algerian documents summing up terms of the complex deal. The U.S. negotiator initialed the upper right-hand corner of each of the more than 20 pages and signed his full name and the date to the last page of each of the three documents.

Nearly two hours after the hurriedly organized initialing ceremony, to which reporters were called at the last minute, the highly technical documents were read out in English for the better part of an hour by Foreign Ministry translator Helal Farida at the International Conference Center in the monumental hotel Aurassi, the late Egyptian president Gamal Abdel Nasser's independence gift to Algeria.

The translator reas in a droning monotone the texts that made no direct reference to how the hostages would be released. The Algerian Foreign Ministry spokesman accompanying her insisted he was authorized to say no more and greeted questions about the two Algerian planes already on their way to Tehran with angry denials that they had left.

A U.S. diplomat who witnessed the amazement with which American reporters were greeting the lack of discussion by the Algerians of the actual hostage release shrugged it off, saying, "It's their show."

It was as if Algeria was determined to squeeze every last drop of suspense out of the situation.

Nevertheless, Christopher expressed nothing but gratitude for the Algerian role, which by all accounts went far beyond that of a pure intermediary acting only, as the Algerians had claimed, as "a mailman." Participants in the talks noted that the Algerian diplomats working in Washington, Algiers and Tehran were being asked to do so in an obvious effort to overcome various difficulties.

Christopher thanked Benyahia for the "great impartiality, discretion and high skill" with which Algerian diplomats "performed this heavy responsibility."

He added that "the American people will always rember this contribution to humanitarian matters by the Algerian people and their leaders."

Algeria's middleman role has undoubtedly served as a boost for Algerian international prestige, serving to reinforce its previously tarnished claim to leadership in the nonaligned bloc of nations. That role had been somewhat dimmed by the country's severe economic development problems in recent years and its appearance of alignment behind Soviet foreign policy.

Two of the documents initialed by Christopher are titled "The Declaration of the Democratic Popular Republic of Algeria" and "The Declaration of the Democratic and Popular Republic of Algeria on Claims Settlement."

This referred to the unfreezing of billions of dollars worth of Iranian assets. The return of the funds is being described as perhaps the largest single financial transaction in history. But a peculiarty of the documents was that it is difficult to establish from reading them exactly how much Iran and the United States calculate Iran's frozen assets total. That was apparently a calculated diplomatic ambiguity.

The third documents, described here as procedural and the only one that was read out in French rather than English, was a list of nine sets of "decrees," roughly the equivalent of U.S. executive orders, to be issued by President Carter to carry out the previous two long documents.

Those "decrees" included the orders necessary for setting up an escrow account in an unnamed central bank, presumably the Bank of England, for transfer of Iranian funds into Algerian safekeeping; for transfer of Iranian assets at the Federal Reserve Bank of New York and In U.S. bank of New York and in U.S. banks and their foregin branches; for transfer of other Iranian assets such as gold bullion and U.S. Treasury bonds; for revocation of U.S. economic sanctions against Iran; for dropping and canceling lawsuits against Iran, and for a U.S. commitment to help Iran identify and recover the assets of the late shah and his family.

In addition to the main operative principle in the documents that Washington commits itself to "restore the financial position of Iran, in so far as possible, to that which existed prior to Nov. 14, 1979," Washington also agreed that its "purpose" is "to terminate all litigation" by private Americans and companies against Iran and settle all claims through binding arbitration.

The complicated enforcement provisions provide for claims to be submitted to a special international arbitration tribunal that will be created with Iranian, American and third-country members in equal numbers.

The agreements also provide for $1 billion worth of Iranian assets to be maintained in the Bank of England escrow account for the settlement of claims sent to the tribunal.

Washington also pledged that "it is and from now on will be the policy of the United States not to intervene, directly or indirectly, polically or militarily, in Iran's internal affairs."

Iran agreed to certify to Algeria "that the 52 U.S. nationals have safely departed from Iran" and the United States agreed that Algeria could then instruct the Central Bank to transfer most of the funds held in escrow to Iran after that. But both Iran and the United States retained the right to cancel their agreement within 72 hours if either side should default on its commitments.