Iran freed the 52 American hotages today after a night of frantic negotiations on a last-minute snag that threatened to scuttle a deal that had been accepted by Washington and Tehran.

On their 444th day of captivity, the Americans were brought to Tehran's Mehrabad International Airport under tight security and put on board one of two Algerian Boeing 727 jetliners that had been standing by in a far corner of the airport since yesterday. The pilot of one of the planes due to fly the Americans out of Iran filed a flight plan at around 4:30 p.m. Tehran time (8 a.m. EST). The second jetliner reportedly was loaded with baggage.

The flight plan reportedly called for the planes to fly over Turkey on their way to Algiers. From there, the hostages were scheduled to be flown aboard U.S. Air Force medical-evacuation planes to West Germany, where they would rest in an American military hospital at Wiesbaden for several days before being repatriated to the United States and their families.

The snag that held up the hostages' release for a nerve-wrecking day and night boiled down to a conflict over the amount of Iranian assets held in U.S. banks and transferable to Iran. The difficulty surfaced when Iran's chief hostagae negotiator accused the United States early today (Tehran time) of adding a surprise document to their hostage release agreement.

The problem was resolved by a change in the document's language accepted by Tehran at about midday in the Iranian capital.

In the meantime, Iran had threatened to set a deadline for taking "more severe action" if Washington did not start the transfer of Iranian assets to a bank account in Britain.

The threat was made by Iranian hostage negotiator Behzad Nabavi before Washington announced that agreement had been reached on all aspects of the negotiations and that banks had been ordered to start moving Iran's frozen assets to London.

According to a Tehran diplomatic source reached by telephone, the dispute apparently was resolved when Nabavi called Algiers around 11:30 a.m. today (3 a.m. EST) and spoke to the Algerian mediators. The call tallied with a White House report that it was informed from Algiers at 3:16 a.m. EST that Iran had accepted a rewritten version of the disputed document.

However, Nabavi continued to espouse a tough line thereafter, apparently for domestic consumption following signs of criticism that Iran had failed to get all it originally demanded from its agreement with the United States.

Nabavi warned in a radio interview broadcast at 2:30 p.m. Tehran time (6 a.m. EST) that Iran had set a deadline of 3 p.m. Tehran time for the United States to transfer Iran's assets to Britain, "or else we will take necessary measures." By the time the taped interview was broadcast, Washington announced that the money had already been transferred to London.

As Iran prepared to send the hostages out of the country, ending the Americans' long ordeal, Tehran radio declared that the hostages' general health was "satisfactory." It said six Algerian doctors who had examined the Americans over a period of 12 hours yesterday said there would be "no point" in the two months of rehabilitation planned by U.S. authorities for the hostages after they returned home, Agence France-Presse reported.

Earlier, Nabavi had issued a statement around 1:30 a.m. Tehran time (about 5 p.m. Monday EST) accusing U.S. banks of "an underhanded maneuver" that delayed the hostages' departure from Iran.

The sudden accusation temporarily stemmed soaring hopes that the hostages at last were on their way out of Iran and put off conclusion of an agreement that Iran's negotiators had defended against critical questioning hours before as a major defeat for "the Great Satan."

According to the official Pars News Agency, which carried Nabavi's charge, he said Algerian intermediaries in Tehran had handed the government an 11-page appendix drafted by American banks to oblige Iran to give up some of its U.S.-held assets.

Officials in Washington immediately denied that they had sought to dupe the Iranians. These officials protrayed the last-minute difficulty as a purely technical one that could be overcome quickly.

Iranian Central Bank Governor Ali Reza Nobari said the problem involved an amendment that would foreclose Iran's right to challenge the amount of its assets held by U.S. banks, but that the mater could be resolved fairly easily by a change in language. By contrast, Nabavi's remarks seemed to reflect a more serious political side of the problem -- fears that the agreement generally might provide ammunition for the government's domestic critics.

In an English-language dispatch, Pars said U.S. banks "have meant to make it binding on Iran to drop any further claims beyond the approximately $8 billion . . . to be escrowed in the British central bank." The agency quoted Nabavi as saying this violated the agreement negotiated in Algiers.

The crux of the dispute appeared to be a discrepancy -- long evident in the two sides' public statements -- on the amount of Iranian assets frozen by the United States. Nabavi said the agreement announced yesterday acknowledged that Tehran and Washington did not agree on the amount and stipulated that this dispute "would have to be settled through the reciprocal banks of the two countries," Pars said.

The agency quoted Nabavi as saying that the unnamed U.S. banks now claimed that $8 billion was "the whole of the assets credited to Iran" and that the agreement announced Monday contained no such appendix.

Pars said the appendix came "at the 17th hour, which even with the utmost optimism could only be viewed as an underhanded maneuver for delaying the final solution of the problem, especially after the U.S. president had issued an order for releasing Iran's assets in the U.S. banks."

"Nabavi said that the government of the Islamic Republic of Iran severely condemned this subterfuge by the U.S. banks and wishes to open the minds of the world's people and especially the minds of the American public to this fact," Pars said.

The chief Iranian hostage negotiator said that under the U.S.-Iranian agreement, the American banks must transfer Iran's assets to the British central bank before the hostages could be freed. As of midnight in Tehran (3:30 p.m. EST Monday), Iran had not received word of this transfer, Nabavi said.

According to Pars, Nabavi "blamed the U.S. banks for needlessly dragging out the issue." He said U.S. officials were negotiating with Algerian intermediaries in Algiers to resolve the problem and that the Algerian government "fully supported Iran's stand."

Nabavi's charges delayed the last-minute efforts to tie up the loose ends of the agreement, which was announcd after weeks of steadily intensifying negotiations among Washington, Tehran and Algiers.

His accusations came after a day of conflicting reports about an imminent end to the 52 captive Americans' 14 1/2-month ordeal.At different times during the day, it was reported that the hostages were about to be flown out of Iran and that technical hitches had developed in setting up an Iranian escrow account in London.

The hitches initially were characterized as relatively minor both in Tehran and Washington, and prepareations went ahead at Tehran's Mehrabad International Airport to received the hostages and get them on their way home.

Two Algerian Boeing 727 jetliners were standing by on the tarmac along with a smaller plane used to bring six Algerian doctors to Tehran Saturday to examine the hostages before their release.

The doctors performed these examinations yesterday, and film of the Americans undergoing the check-ups and eating lunch was shown on Iranian television later in the day.

According to Iranian sources reached by telephone in Tehran baggage was apparently loaded on one of the Algerian jetliners in preparation for their departure, and security forces around the terminal were strengthened.

These sources said the plan initially was to bring the hostages to the airport and send them off in the early hours of Tuesday Tehran time when the terminal could be more easily cleared.

Earlier yesterday, Iranian officials had proclaimed the agreement with the United States as a major defeat for "the Great Satan" amid indications of pressures building in Tehran to justify Iranian concessions.

Nabavi hailed the accord as a triumph for Iran at a news conference and on a radio talk show. But the nature of some of the questions from Iranians and the defensiveness of his replies suggested that the Iranian negotiators were under some pressure to fend off potential criticism from domestical political adversaries.

In the first question, an unidentified listener said, "Certain people believe that you have solved the hostage issue from a position of weakness. They claim that America is holding the trump card."

Nabavi replied that such critics either had "an ulterior motive" or were "unrealistic." He then went on to say, "God willing, once we present the outcome of our work, then everyone will realize that it was not from a position of weakness. Rather, it was from a position of strength and based on victory."

The question appeared to reflect earlier criticism from the camp of President Abol Hassan Bani-Sadr, who has argued that by refusing to allow the hostages' release early last year, his Moslem fundamentalist rivals eventually weakened Iran's position in negotiating on the issue.

The government of Prime Minister Mohammed Ali Rajai also faces a potential backlash from another quarter, the Islamic hard-liners who opposed a settlement of the issue when Iran's Majlis, or parliament, was debating four conditions for the Americans' release.

In an apparent effort to spread the responsibility for the agreement with the United States, Nabavi repeatedly referred to the parliament's authorization for the government to seek a settlement.

Another Iranian questioner put Nabavi on the spot by asking if he would personally take responsibility for the agreement if there were any public criticism of it. Nabavi replied that both he and the government stood behind everything in the accord.