Talks over postponing Nicaragua's election date broke down over opposition leaders' refusal to sign a letter to President Reagan urging a change in U.S. policy toward Nicaragua, Nicaraguan junta leader Daniel Ortega said today.

In a 90-minute interview before the opening of the U.N. General Assembly session, where he is to speak Tuesday, Ortega said Nicaragua's presidential election will occur Nov. 4 as scheduled.

"We were flexible on the date until a few days ago," he said. "The elections are going to take place on Nov 4. At this point we cannot continue to play with the date."

In Managua, however, an opposition leader told special correspondent John Lantigua that a major opposition group was close to signing an agreement with the ruling Sandinistas that would provide for a postponement of the election date if a cease-fire can be arranged with rebel groups based in Honduras and Costa Rica. A Sandinista source at the highest level confirmed the terms of the agreement, which was worked out with the mediation of leaders of the Socialist International, but expressed some doubt that the accord could be implemented.

Agustin Jarquin, a director of the opposition Democratic Coordinator, said the group's presidential candidate Arturo Cruz had worked out an agreement with Sandinista representatives at a meeting of the Socialist International in Rio de Janeiro.

The Sandinista source in Managua said the parties making up the Coordinator would have to register their candidates by midnight Monday. Instead, representatives of the three parties went to the electoral tribunal and requested an extension of the deadline, and Jarquin said his group would require new evidence that the Sandinistas would permit a free election before it would agree to participate.

Sources in Washington monitoring the talks said discussions were still continuing and agreement could come at any time before voting day.

Ortega, 38, also said he was "concerned" about Democratic presidential candidate Walter Mondale's new tougher line on Nicaragua, and charged that U.S. policy is now one of "genocide" against the Nicaraguan people.

Ortega, eating very little at a breakfast in his heavily guarded hotel suite, said his leftist Sandinista government proposed Sept. 17 that Cruz register as a candidate "without preconditions" and then join seven other opposition parties in drafting a letter to Reagan.

The letter would ask Reagan "to stop his aggression against Nicaragua so the election could take place in peace," Ortega said, referring to the three-year-old effort by U.S.-backed rebels to overthrow the Sandinista government.

In return for the letter, the Sandinistas "would study the possibility of holding the elections the last Sunday in November," a three-week postponement, Ortega said.

The idea of the letter was first raised publicly in July when Cruz's coalition refused to register its candidates for the election before the original deadline set by the Sandinistas. Cruz said at a news conference July 25 that he had rejected the suggestion that he sign a petition to Reagan and the U.S. Congress as "absolutely ridiculous."

Ortega said the Sandinistas proposed the letter on Sept. 17, and on Sept. 21 Cruz responded in a letter to President Belisario Betancur of Colombia, who had been acting as go-between, with new conditions and a proposed date of Feb. 24.

"That was really something absurd," Ortega said.

Ortega said that in a long, private talk with an unnamed member of the Sandinista government, Cruz had said he favored the Nov. 4 date but was overruled by the conservative businessmen in the Democratic Coordinator.

"He is acting in good faith but he is a victim and an instrument of the policy of aggression against Nicaragua, Ortega said. "He said he was powerless and couldn't do much against the Coordinator." Ortega said he continues to fear U.S. military action if Reagan is reelected. He added that he was "very concerned" by Mondale's recent remark favoring a "quarantine" of Nicaragua if the Sandinistas continue to aid leftist rebels in El Salvador. He blamed it on "a belligerent climate" in the United States.

The antigovernment rebels known as contras, or counterrevolutionaries, have inflicted "enormous damage" in Nicaragua, Ortega said.

Ortega said he would tell the General Assembly that Nicaragua "is undergoing a true genocide" that has not ended with the refusal of Congress to provide more funds for the contras.

Lantigua reported the following from Managua:

Opposition leader Jarquin said that officials of the Socialist International had agreed to approach the leaders of U.S.-funded rebel organizations and U.S. officials in order to bring about a cease-fire by Oct. 25. If a cease-fire goes into effect by that date, the Sandinistas have agreed to postpone the election until Jan. 25.

The agreement on the table in Rio de Janeiro, according to Jarquin, calls for Cruz to register as a candidate as soon as the agreement is signed and to begin his campaign. The Sandinistas, said Jarquin, have assured Cruz they will meet all his demands for a fair campaign: no press censorship, no harassment of him and other opposition candidates by Sandinista mobs and access to state-run television and radio stations.

In return, the leaders of the Socialist International have agreed to work out the cease-fire. According to Jarquin, if the cease-fire cannot be arranged, the Sandinistas will proceed with their present electoral calender, the voting will be Nov. 4 and the Coordinator will not participate.

He said if a cease-fire is implemented, the Sandinistas have agreed to postpone elections until Jan. 25. He said if the Sandinistas at any time broke the agreement regarding a fair and clean campaign, the Coordinator would withdraw.

One of the principal mediators who sat in on Cruz's meetings in Rio with Sandinista leader Bayardo Arce was Carlos Andres Perez, former president of Venezuela and a top official in the Socialist International. Sources said Perez, Betancur and former West German chancellor Willy Brandt, another Socialist International leader, would be the "guarantors" of the agreement.