Nicaragua has alleged that former ambassador to the United States Francisco Fiallos, who resigned three weeks ago protesting what he said was the "tyranny" of leftist "radicals" within his government, withdrew without authorization $668,000 deposited in Riggs Bank from the April sale of the long-unused ambassadorial residence here.

Foreign Minister Miguel D'Escoto said yesterday in Managua that he believed the 36-year-old former diplomat had taken the money, which Riggs records show Fiallos withdrew in three cash installments between September and last month..

Fiallos, reached by telephone in Costa Rica, called the charges "a slanderous lie" designed to discredit him. Fiallos said he withdrew the money in question acting under coded instructions sent to him by the Nicaraguan Foreign Ministry. He said he had been instructed to turn the cash over to "someone sent for it" on a confidential mission from Managua and had done so.

D'Escoto and embassy officials in Washington said they suspected the money was gone soon after Fiallos announced his resignation Dec. 20.

Although they said Riggs confirmed the account was empty in late December, they said they withheld the information until Fiallos, who had disappeared from public view, could be found for an explanation. One source close to the unfolding of events said the government feared it "wouldn't be believed," since any denunciation of Fiallos would be looked upon as retribution for Fiallos' attacks on the Sandinistas.

On Thursday night, however, Fiallos contacted D'Escoto by telephone from San Jose, Costa Rica, and, according to D'Escoto, first denied having anything to do with the money. When confronted with details from bank documents, D'Escoto said, Fiallos responded only that if he had the money he would give it to the anti-Sandinista movement of former Nicaraguan rebel leader Eden Pastora.

Yesterday, D'Escoto brought up the issue for the first time publicly in a meeting in Managua with a visiting group of American academics and journalists.

Pastora said from Costa Rica that both he and Fiallos had spoken to D'Escoto by telephone Thursday. Fiallos surfaced publicly yesterday at a San Jose press conference where, with Pastora at his side, he pledged allegiance to Pastora's Revolutionary Democratic Alliance. The alliance is one of two principal groups active in military and political opposition to Nicaragua's ruling Sandinista National Liberation Front.

Pastora, who in recent months has complained that his movement's finances were "down to the bottom of the pocket" and accused the U.S. Central Intelligence Agency of helping another Nicaraguan opposition group made up of supporters of the late dictator Anastasio Somoza, said he knew "absolutely nothing" about the money Fiallos is alleged to have taken. Speaking from his Costa Rican headquarters, he repeated Fiallos' charges that the Sandinistas manufactured the story to discredit their opponents.

Pastora and Fiallos said the Sandinistas had tried in the past to cast aspersions on a number of officials who have defected from the government, including Pastora, former Central Bank head Alfredo Cesar and Fiallos' predecessor Arturo Cruz, who also resigned the ambassadorship.

Others, however, said Fiallos' statement appeared unlikely. Cruz, who lives in Washington and is a Pastora partisan, said it "does not make sense." Expressing hope that the Sandinista version would not be corroborated, Cruz said if true it would "be terrible for Eden" Pastora and could "destroy the only viable option" for a new, or at least revised, Nicaraguan government along the more pluralistic lines Pastora has proposed.

The indirect subject of the current conflict, the 2.1-acre former Nicaraguan ambassadorial residence at 3200 Ellicott St. NW, was torn down shortly after its sale last April, for about $860,000, to a Washington development company.

But the once-elegant mansion had been cause for controversy since the Sandinista victory against Somoza in July 1979. Initially, the Somoza government's ambassador, Guillermo Sevilla-Sacasa, who was at that time dean of the Washington diplomatic corps and had lived in the building for decades, refused to vacate the house and turn it over to the government of the victorious Sandinistas, provoking a colorful diplomatic argument. The Sandinistas subsequently boarded and closed the building. In November 1981, the uninsured house was gutted by a fire that investigators said at the time they believed had been deliberately set.

When Fiallos, a Harvard-trained attorney and Sandinista Front member, arrived in Washington a few months after Cruz's December 1981 resignation, he followed government instructions to put the house on the market. According to District of Columbia real estate records, it was sold April 6. Fiallos signed the deed on behalf of the embassy, and $820,530 was placed, under his signature, in a Riggs savings account and Eurodollar call deposit at 13 percent interest.

According to Riggs records and copies of cashier's checks obtained by The Washington Post, a series of withdrawals, which other embassy officials said were to cover operating expenses and requests from the Foreign Ministry, were made between April and September, leaving a balance on Sept. 7 of $687,944.09.

On Sept. 13, according to the bank records and a Jan. 3 letter to the embassy from the bank, "Ambassador Fiallos withdrew $300,000." On Sept. 16, the letter said, "Ambassador Fiallos withdrew $350,000." There is no record of a cashier's check or other instrument being issued. Fiallos said yesterday the money was withdrawn in cash on government instructions and turned over to a "confidential messenger."

Five weeks after that withdrawal, Fiallos was instructed to withdraw and send to his govenment $80,000 for a new diplomatic residence in Costa Rica, D'Escoto said. By that time the balance was only $48,403.65. On Oct. 21, "Ambassador Fiallos deposited $50,000 cash" which allowed the account to cover a cashier's check for $80,000. The next, and final, transaction came on Dec. 7, when the bank "disbursed cash" to Fiallos equalling the balance of $18,253.04 and closed the account.

Fiallos said he does not remember the instructions authorizing the final withdrawal. Repeating his charge that the Sandinistas were "lying, and making things up," he said "you are asking me things I don't have at the tip of my tongue" since he did not have the pertinent documents with him. Fiallos said he had "no idea" of why the two cash withdrawals totalling $650,000 were ordered by Managua in September and declined to identify to whom he had turned over the money.

"The only thing I can say about all of this," Fiallos said of the government allegations, "is that it is a fabrication, an invention, the same thing the Polish government said about Solidarity union leader Lech Walesa." Allegations concerning the money had not yet surfaced and were not mentioned when Fiallos held his San Jose news conference yesterday. At that time, Fiallos said he "called on the Nicaraguan people to prepare for the struggle" against the Sandinistas and said Pastora's alliance "represents the real aspirations of Nicaragua" for a democratic government.

When he announced his resignation Dec. 20, Fiallos said the immediate cause was Sandinista censorship of an interview he had granted La Prensa, the opposition newspaper in Nicaragua, in which he had issued a call for political pluralism and free elections. The interview had been scheduled to run in the Dec. 8 edition of the newspaper.

On Dec. 22, two days after the resignation, Saul Arana, a Foreign Ministry official who had travelled here from Nicaragua, and acting embassy Charge d'Affaires Manuel Cordero, visited Fiallos at his apartment and asked, among other things, about the bank account, according to a source close to the government who said he also was present at the meeting. Fiallos, the source said, promised a full accounting of this and other outstanding embassy matters "within a few days."

Later, the source said, they asked Riggs to replace Fiallos' name with Cordero's on the account. It was not until Dec. 28, Cordero said, that the embassy was informed the account was empty. By that time, he said, they could not locate Fiallos.