MANAGUA, NICARAGUA, DEC. 13 -- President Daniel Ortega said today that plans disclosed yesterday by his brother, Nicaragua's defense minister, for a major armed forces buildup were merely a military "proposal" to the government that had not been accepted. He asserted that Nicaragua would have "a modest Army" after the current civil war ends.

In an interview after a speech to a union assembly, Ortega sought to play down the speech by his brother, Gen. Humberto Ortega, yesterday in the same forum. President Ortega stressed repeatedly that the Sandinista government was respecting a Central American peace accord and was willing to negotiate arms reductions with its neighbors and the United States.

"We have talked of having all our population learn to handle arms to be ready to defend the country, but not of organizing an army of 600,000 men, because this would not make sense," Ortega said. "This country could not support an army of that magnitude."

President Ortega's account of Nicaragua's military plans contrasted sharply with the statements yesterday by his brother that appeared to confirm revelations by a Sandinista military defector, Maj. Roger Miranda Bengoechea, regarding military cooperation protocols between Nicaragua and the Soviet Union.

Asked about his brother's account of a 15-year plan to build up the Sandinista regular armed forces, reserves and militias to a total strength of 600,000 by 1995, Ortega said: "No, this plan does not exist. The statement that Gen. Ortega made yesterday is in the context of the war of the people, if this war continues or intensifies. This is a proposal of the Army for the government, but it has not been accepted by the government of Nicaragua."

The president said that what the government had in mind was a defense system such as exists in Switzerland, in which "the entire population is ready to defend the country."

At another point, Ortega said that the figure of 600,000 was contemplated largely in terms of reserves that could be mobilized to combat an invasion.

The president said Gen. Ortega's statements yesterday were "linked to the time of war that Nicaragua is living through," a period marked by U.S. aid for the Nicaraguan rebels, known as contras, and the acquisition by neighboring Honduras of F5 fighter planes from the United States.

"We have always thought that once this war ends, the Army here should be a modest Army," Ortega said.

The Sandinista leader made the statements after a speech of more than two hours in which he told an assembly of union delegates that the Sandinista National Liberation Front would not give up "revolutionary power" or allow domestic opposition groups to become an "internal front" of the contras.

Issuing a stern warning to parties and news media of "the right" to "act with responsibility," Ortega said the Sandinista government would "mobilize the workers of all companies to go to combat" if necessary, but he made no mention in his own speech of his brother's remarks yesterday.

After the speech, Ortega's aides invited this reporter for an interview at the president's request to clarify and explain the defense minister's remarks.

In particular, President Ortega's version conflicted with the description in the alleged SovietNicaraguan protocols of a Sandinista military buildup independent of the current war with the contras and the regional peace efforts. One passage in the alleged protocols estimates that the contras will suffer "total defeat during the period 1988-90" and states that the subsequent goal is to be able to defeat a U.S. invasion.

Asked whether the agreements mentioned by Miranda exist, Ortega said Nicaragua has relations "of a military type" with Moscow. "We are not in an agreement as such in the terms that Miranda tried to present, but they are accords that Nicaragua has reached as to assistance from the Soviets and that cover the necessities for the defense of the country," Ortega said.

He added, "The statements of Miranda are mixtures of elements that have a certain truth with other elements that are totally false."

Miranda, who was a top aide to Humberto Ortega, said Thursday in his first interview since his defection that top Sandinista officials were involved in drug trafficking, diversion of government funds to private companies and bank accounts, and the purchase of luxury items in Panama for other officials and their families.

Miranda charged that since the July 1979 revolution that brought the Sandinistas to power, Humberto Ortega has diverted more than $1.4 million from his ministry's budget into a Swiss bank account in the name of a company called Representaciones Multilaterales del Caribe.

In his speech yesterday, Humberto Ortega said drug and corruption charges against Sandinista officials were "calumnies." He did not refer to Miranda by name or discuss the allegations in detail. His references to these and other accusations by Miranda apparently came in response to written questions submitted to the Defense Ministry by The Washington Post along with a request for an interview. The ministry requested the questions, then declined to answer them.

In the interview today, President Ortega said the drug and bank account allegations were nothing new, but were being trotted out now as part of a Reagan administration effort to get Congress to approve new aid for the contras. He declined to comment specifically when asked about the companies mentioned by Miranda.

"Yes, the government of Nicaragua has different companies in different places, and they are legal," Ortega said. He described these as related to Nicaraguan efforts to circumvent a U.S. trade embargo by, for example, obtaining American spare parts through Panama. "Yes, this exists, but not in the form that Miranda is saying to damage the image" of Sandinista officials, Ortega said.

Such items as radios and cars are supplied to government and military officials "so they can carry out their funcions," Ortega said. "There is nothing illegal in all this."

Questioned about Miranda's charge that Nicaragua has trained Salvadoran guerrillas to use Soviet shoulder-fired antiaircraft missiles, Ortega would say only that "those who have received missiles are the contras, and we have mentioned the danger that exists that these missiles will fall into the hands of other armed groups in Central America and Latin America."

Regarding Nicaragua's plans, mentioned in the alleged protocols with the Soviet Union, to acquire MiG21 fighters, Ortega said: "Nicaragua has the right to possess these planes," although it would be better if all such aircraft "disappear from Central America." He added, "In this situation we need that plane because it is the only thing that can intercept contra supply flights."

Ortega said the presentation of Miranda was a "desperate effort by President Reagan" to get more contra aid, which he said would "prolong the war" and violate the Central American peace plan. But he said Miranda's appearance also reflected a U.S. plan for the "Chileanization" of Nicaragua through "internal destablization." The United States supported opposition groups during the Chilean rule of Marxist president Salvador Allende.

It was in connection with the charge of destablization that, in his speech, President Ortega strongly cautioned opposition parties and media to act responsibly -- a statement taken as a warning not to make too much of the Miranda disclosures.

Wearing an olive-green military uniform as he addressed about 600 union delegates in the Olof Palme convention center here, Ortega said, "We cannot accept a so-called civic opposition that becomes a representative of counterrevolution and imperialism in our country . . . . The government and the Sandinista Front can have a little more patience, but the people can lose patience very rapidly."