Nicaragua's deposed president, Anastasio Somoza, said here today he had nothing to do with his successor's shortlived plan to remain in office despite an agreement to transfer power to the rebel Sandinistas.

During a news conference in his $500,000 house, Somoza called Francisco Urcuyo's announced intention to fill tsomoza's term until 1981 a "trememdous political mistake. We had given our word that the steps were going to be such-and-such."

He said, however, that Urcuyo's departure, after 36 hours in office, did follow a telephone call from Somoza. Somoza told reporters gathered in his home, formerly the residence of the archibishop of Miami, that he pointed out to Urcuyo, "Let's be realistic, let's keep our word on this matter," and after that everything fell into place."

Somoza claimed he did not mention to Urcuyo Carter administration threats "from the highest level" that he, Somoza, might be exiled from the United States if Urcuyo did not step down as arranged.

The former Nicaraguan president, whose family's dynasty controlled the Central American nation for 42 years after initial maneuverings by the United States, repeatedly referred to the U.S.-arranged transfer of a government as though it were a business transaction. Deputy Secretary of State Warren Christopher, Somoza said, "advised me if the deal went through I was welcome to the United States, but if the deal did not go through I was not...,"

On other points Somoza:

Said he tape recorded key conversations with U.S. officials as a "record for history."

Claimed that the United States discussed logistical support for his National Guard after "a lapse of time" so the guard could be a stabilizing force. The United States advocated for a time a combining of National Guard and Sandinista forces after a change in government.

Said his family's fortune was about $100 million, not $500 million as some have claimed and that $80 million in riches were left behind in Nicaragua. Somoza has said he declared to U.S. officials he was bringing $150,000 into the country when he arrived.

Challenged the United States and Latin American countries to "make good" on carrying out the Organization of American States' resolution calling for a democratic government in Nicaragua and enforcement of human rights, early elections and money for reconstruction.

"It is not a democratic government," Somoza said of the Sandinistas. "It is a communist junta dominated by communists."

Somoza also said he "might contemplate" supporting a counterrevolution against any communist government in Nicaragua. But he added that he is "not trying to start up anything while I am here in the United States because I understand the law." He said his plans include a three-month vacation in another country where he will "not be blamed for things."

Somoza's home on Sunset Island is currently under guard by U.S. diplomatic security forces from Washington. CAPTION: Picture, Somoza: "We had given our word that the steps were going to be such-and-such." AP