President Daniel Ortega announced tonight that Nicaragua will send home 100 Cuban military advisers and declared an "indefinite moratorium" on acquiring new arms systems, including "interceptor aircraft," as part of a series of unilateral steps designed to stimulate peace talks within Central America and with the United States.

In a statement he read to reporters and diplomats here, Ortega charged the Reagan administration with blocking bilateral and regional talks and with "aggravating the climate of tension in the region through a campaign of calumnies and lies" about Nicaragua's leftist Sandinista government.

For this reason, he said, Nicaragua had decided to take "unilateral initiatives and decisions" that he was announcing "to the governments, parliaments, international organizations and people of the world."

Initial White House reaction to Ortega's statement was subdued. "We will take a look at these proposals," White House Deputy Press Secretary Robert Sims told Washington Post reporter Lou Cannon in Washington, "but we want to withhold comment until we've had a chance to study them." The State Department issued a similar statement.

Administration officials told Cannon that they did not expect dramatic changes in the U.S. position toward Nicaragua as a result of the new Nicaraguan offer but implied that they would keep the door open.

In two numbered points of his statement, which was distributed here, Ortega said that the first 50 Cuban "military instructors" would be sent home in May, with the others to follow later. Although Nicaragua has said that approximately 4,000 Cubans are present here, it has described most of them as teachers, doctors and technicians. Cuban President Fidel Castro and Nicaraguan officials have said there are 200 Cuban military advisers in Nicaragua, while the Reagan administration has charged that there are as many as 8,000 Cubans here, 2,000 of them military advisers.

However western diplomats here said today that the number of Cuban military advisers was between 300 and 500.

The second point said that "we have decided on the declaration of an indefinite moratorium in the acquisition of new arms systems, as well as those interceptor aircraft required for the completion of the country's existing antiaircraft system."

The administration has charged that Nicaragua plans to acquire sophisticated MiG jet fighters from the Soviet Union. While the Sandinistas have said previously that they had no current plans for such aircraft, the statements always have included their insistence that they "reserve the right" to receive them.

Countering charges by the administration that Nicaragua is a base for Soviet-backed aggression in the hemisphere and is destined to become a Soviet military outpost, Ortega said: "Nicaragua does not form part of any bloc, and it does not have military alliances with anyone . . . . Nicaragua is not and never will be an aggressor country."

Later in the statement he said: "In the face of the pretexts and unscrupulous manifestations of all types alleged by the U.S. government in respect to its security, Nicaragua reaffirms again that it is not and never will become a military base of any foreign country or power."

The statement read tonight by Ortega originally was scheduled to be released Friday in Montevideo, Uruguay, where Ortega is to attend the inauguration of President Julio Sanguinetti. The government decided to go ahead with the announcement tonight "to rob Mr. Reagan of his pretexts . . . . for his unjustifiable policies," according to Alejandro Bendana, the first secretary of the Foreign Ministry.

"The ball is once again in Mr. Reagan's court," Bendana said.

In Montevideo, Nicaraguan Information Minister Manuel Espinoza said Ortega would be willing to meet with Secretary of State George P. Shultz while the two were in that city for the inauguration, Reuter reported.

The Nicaraguan initiative comes as the Reagan administration is engaged in an all-out effort against heavy opposition to gain congressional approval of renewed funding for CIA-backed anti-Sandinista rebels.

This week, the Sandinistas launched their own effort in Washington, headed by Deputy Foreign Minister Victor Tinoco, to lobby members of Congress to vote against the funding. Tinoco today delivered to a number of congressmen a copy of Ortega's statement, which included an official invitation for a bipartisan U.S. congressional delegation to visit his country "without restrictions" to see that military preparations there are "strictly defensive."

This afternoon, before Ortega's statement was made public, the administration accused the Sandinistas of waging a "show-and-tell propaganda offensive."

Referring to the congressional invitation, which was announced separately in Managua last night, White House spokesman Larry Speakes heaped scorn on Ortega's "so-called peace initiative." In words similar to those used recently by Reagan, Speakes said that while Ortega was talking peace, the Nicaraguan government was consolidating "a totalitarian Marxist-Leninist state in this hemisphere."

"We would welcome any new peace proposal that would meet our concerns," Speakes said. "They the Sandinistas know about concerns over their military buildup, their subversion of their neighbors, their Cuban and Soviet Bloc ties and their repression of their people, the church, the press and the democratic elements within the country. In our view, an appropriate peace offensive by them would be one that contains concrete actions to meet these concerns rather than a show- and-tell propaganda offensive."

The Nicaraguan statement said, "our relations of friendship and growing cooperation with socialist countries and in particular with the Soviet Union and Cuba are an expression of the genuine nonalignment of Nicaragua. Respectful friendship with all states is in accord with our independence, sovereignty and self-determination."

It added that revolutions such as the Sandinista one "cannot and should not be exported."

Ortega did not comment tonight to reporters beyond reading the statement, after which the Sandinista anthem was played and he left the room. The eight-page statement spoke of unilateral actions directed not only toward promoting continuation of the Contadora regional peace talks but also "to the U.S. government's return to the the Manzanillo talks and the end of solicitation of funds from the North American Congress and the end of hostile policies toward Nicaragua."

The Contadora group is composed of the governments of Mexico, Venezuela, Panama and Colombia and named after the Panamanian island where they first met in January 1983 to seek peaceful solutions to Central American conflicts. Since then, they have held a series of negotiating sessions with five Central American governments -- Nicaragua, El Salvador, Honduras, Guatemala and Costa Rica -- and last September drafted a "regional peace and cooperation treaty" calling for the withdrawal of all foreign military bases, advisers and troops from the region and the end of foreign military exercises.

Although Nicaragua announced its willingness to sign the treaty, the United States said it disapproved of some segments, and the other four Central American governments refused to sign it.

Tonight's statement said Nicaragua "is confident that our initiative will stimulate the Central American governments toward the political willingness to subscribe to the Contadora proposals."

Nicaragua last summer began bilateral talks with the United States in the Mexican city of Manzanillo. After nine rounds, the Reagan administration last month suspended the talks on grounds of Nicaraguan intransigence. Nicaragua has said, however, that it had offered to accommodate U.S. objections to the Contadora draft treaty, and the administration walked out of the discussions rather than accept Nicaraguan concessions.

There have been no indications that the administration is willing to resume the bilateral meetings. The separate Contadora regional talks broke down earlier this month following a dispute between Nicaragua and Costa Rica. Costa Rica charged that the Nicaraguans had arrested a Nicaraguan man who had sought political asylum in its Managua embassy. Nicaragua said the man, Jose Urbina Lara, was a "counterrevolutionary," and was not on embassy grounds when arrested.

Costa Rica refused to attend the meeting, and the other three Central American governments followed suit.

Tonight, Ortega said that Nicaragua will "take immediate practical steps to overcome the obstacle, the pretext, that made the Feb. 14 [Contadora] meeting impossible." Foreign Ministry official Bendana said later that this meant Nicaragua was now prepared to hand Urbina Lara over to the Costa Rican government.

The statement stressed the importance Nicaragua puts on relations with Western European countries, saying Managua hopes to "promote and stimulate more European participation and cooperation in Central America."