Nicaragua's ambassador to the United States said yesterday that he has resigned from his government, and from the Sandinista National Liberation Front that runs it, because "radical" Sandinistas are turning the leftist government into a "tyranny" that no longer listens to its own people.

Francisco Fiallos, 36, said he made his decision after failing at numerous attempts to persuade the Sandinistas that they are "losing public support" because of their increasing authoritarianism under "state of emergency" regulations imposed in March.

"There is growing discontent" in the country, he said. "Production is bad, and there is no feeling among the people that they are living under a system of law and judicial protections. Decisions are made by ever fewer people, with ever less consultation." At first, he said, "they listened to me. But then they lost interest in listening. Now, they hear, but they do not listen."

Fiallos is one of a growing number of high-level Nicaraguan officials, and the second ambassador to Washington in little more than a year, to resign because of political differences with the Sandinista government. His predecessor at the Nicaraguan Embassy here, banker Arturo Cruz, has since joined a group of Nicaraguan exiles opposing the Sandinistas on grounds that Sandinista Marxists and their Cuban and eastern Bloc allies have perverted the democratic goals of the Nicaraguan revolution.

Fiallos, a Harvard-trained attorney who joined the Sandinista Front in 1977 as an underground supporter in its struggle to overthrow dictator Anastasio Somoza, said in an interview yesterday, "For the moment, what I want to do is rest and think." But he acknowledged that he has been in touch with Cruz and others of his group, led by former Sandinista guerrilla hero Eden Pastora, and sees joining them as "an option."

The Nicaraguan government, in a brief announcement Saturday, said that Fiallos was being reassigned along with a number of diplomats. But Fiallos said that he had informed Foreign Minister Miguel D'Escoto of his decision to resign on Dec. 12, two days after the government prohibited publication of remarks calling for political pluralism and free elections he had made to La Prensa, the country's only wide-circulation opposition newspaper.

Fiallos' resignation is likely to weaken further the Sandinistas' credibility among a diminishing group of supporters in the West who see them as increasingly authoritarian and unwilling to allow political freedoms.

Much of what remains of their support among Socialist governments in western Europe and among some Latin American countries, according to officials of those governments, is more a reflection of those countries' opposition to Reagan administration hostility to Nicaragua than active support for the Sandinista Front. The Europeans, who support some aspects of Sandinista policy, argue that there is no possibility of tempering the Nicaragua government's militarism as long as it is threatened by armed opposition groups supported by outside powers.

Like Cruz, Fiallos coupled his criticism of the Sandinistas with a denunciation of Reagan administration economic and political pressure against them, as well as covert support for exile groups led by defeated Somoza soldiers trying to overthrow them militarily.

"I stand by everything I ever said about U.S. policy," Fiallos said. The Sandinista "radicals," he said, "have Ronald Reagan and this administration on their side" because Reagan support for the hated Somocistas strengthens the radicals' hand.

"This is the most important point," he said. "The Reagan policy has to change, and let Nicaragua alone to solve its own problems. It is a Nicaraguan problem, a Sandinista problem." The Sandinista government repeatedly has charged that its increasing militarization and centralized control is a response to what they see as a security threat from the Reagan administration.

As have other Sandinista dissidents and defectors, Fiallos also said he was disturbed by the strong presence of Cuban and other eastern Bloc advisers in Nicaragua. These include what other informed Nicaraguan sources have said are as many as two dozen Bulgarians.

The Bulgarians, according to these sources, work as advisers in the planning and economy ministries. Presumably sent by the Soviet Union, their task is believed to be as much to watch the Cubans, whose independent intentions in Latin America the Soviets sometimes mistrust, as the Nicaraguans.

But Fiallos also said that while direct Cuban contribution in high-level government decisions was often talked about, especially in the United States, he had not seen it. He gave some credence to reports that Cuban President Fidel Castro at times had sought to be a moderating influence among the Sandinistas.

In describing the current internal political balance within the Sandinista Front and its nine-man directorate, Fiallos said he believed there is a division between what he called the "radicals" and the "pragmatists."

He declined to comment "at this time" on which Sandinista leaders he would put in each category. Other Nicaraguan dissidents have listed Planning Minister Henry Ruiz and legislative leader Bayardo Arce, as well as Interior Minister Tomas Borge, as "radicals," and have named Agriculture Minister Jaime Wheelock as the chief "pragmatist" among the nine.

But, according to Fiallos, "they all have the same line" while seeing different ways of achieving the same goal. "Some of them are more pragmatic, but none has the power to act alone. They are prisoners of themselves."

When confronted in private meetings with their apparent loss of domestic and international support, he said, "They make excuses. Some of them deceive themselves, others have a scenario in mind but refuse to see that reality is different than what they believe.

"No revolutionary process is perfect," he added. But beginning with the postponement of promised elections until at least 1985, and "problems with the emergency law" that prohibits activity by non-Sandinista political parties -- decreed after a series of exile raids -- "things got worse."

Fiallos arrived in Washington last February, after serving as deputy foreign minister following the July 1979 Sandinista victory and later as Nicaraguan ambassador to Canada. In June, he returned to Nicaragua for a visit, he said, and was shocked by increasing press censorship and an incident in which a Managua priest was beaten by "revolutionary" youths and his church closed.

"There were always more doubts about elections, and the political system was ever more closed. There were confrontations everywhere and, like little boys, the Sandinista leadership kept falling into traps set for them by the Reagan administration."

Last month, Fiallos said, he visited his home town of Matagalpa, in north central Nicaragua, and found the peasants afraid and anxious over "the increasing arbitrariness" of local government representatives. He decided to answer a request from La Prensa for a written interview, which he sent from Washington. "I decided that if they wouldn't let them print it, I would resign."

Fiallos said he had planned to return to Nicaragua last Saturday, but was advised by friends of "some dangers" including what he said was the arrest for "counterrevolutionary activities" of a deputy justice minister who resigned several weeks ago.

"What is important for Nicaragua," Fiallos said, "is that everyone leave us alone. The Cubans, the Soviets, the Bulgarians, the Americans. Everyone should get out and leave us alone."