MANAGUA, NICARAGUA, MARCH 3 -- Nicaraguan rebel leaders today rejected President Daniel Ortega's dismissal of Cardinal Miguel Obando y Bravo as mediator in a cease-fire negotiation, consenting to a top-level meeting with Sandinista officials only if Obando participates as "moderator and observer."

The five directors of the Nicaraguan Resistance, the alliance of the rebels known as contras, announced in Miami that Adolfo Calero, one of their members, would represent them at the talks, which Ortega proposed for March 9-11 in the southern Nicaraguan border town of Sapoa.

The contras insisted that the discussions should be based on a proposal that Obando presented during the last round of cease-fire talks, held in mid-February in Guatemala City. The cardinal's proposal, which called for political reforms in Nicaragua, was accepted "in principle" by both sides -- but since then the Sandinista government has backed away from it.

Contra leader Pedro Joaquin Chamorro said that in three telephone conversations starting last night with Nicaraguan Resistance directors, the cardinal said he could continue in the discussions only if both sides agreed.

"The exclusion of the cardinal at this stage is completely unacceptable to us," Chamorro said. "He is the only witness who is trusted by all the Nicaraguan people."

In this capital, a range of political leaders also expressed dismay at Obando's abrupt removal.

"If the talks were stalled with the cardinal present, in his absence they're headed for certain failure," said Mauricio Diaz, head of the left-of-center Popular Social Christian Party and a member with Obando of the four-man National Reconciliation Commission named by Ortega to monitor the progress of the peace accords.

"Firing the cardinal is a sign that the Sandinista Front is going to make things very hard inside Nicaragua," said Carlos Salgado, general secretary of a left-wing trade union confederation.

A meeting of leftist and moderate opposition unions, including Salgado's, was disrupted this morning by about 150 Sandinistas carrying pistols and metal bars. Shouting and shoving, they stormed into a union headquarters, slashed posters and stole two megaphones as Sandinista police looked on.

Ortega, with his unexpected proposal, named his defense minister and brother, Gen. Humberto Ortega, to head the delegation to the Sapoa talks. The team also would include the Army's second highest officer, Chief of Staff Gen. Joaquin Cuadra, and, the government announced today, Lenin Cerna, the much-feared head of the State Security police.

By putting the talks in the hands of the military, Ortega was giving clear notice that the Sandinistas' agenda will be limited to technical aspects of a cease-fire -- such as choosing of zones and observers and deciding how the contras would be supplied with food and medicine during the truce.

In a letter that Ortega hand-delivered to Obando last night, the president recalled that the five Central American presidents, in their January summit in Costa Rica, affirmed that any cease-fire talks under their regional peace plan should be "separate from a political dialogue to be conducted with the unarmed political opposition parties."

Nicaraguan Resistance leaders insist that the Sandinistas must make broad changes -- such as separating the Sandinista party from the national Army, ending the compulsory draft and disbanding the rural militias -- before they will lay down their arms.

Ortega apparently opted to spring the new proposal of high-level direct talks to cushion the international impact of his removing Obando as mediator before the next round of talks. Since the most recent round was suspended by the cardinal Feb. 19, he made it clear that he planned to exercise independent authority in the negotiations and to press the Sandinistas to move forward on the political front.

In a Feb. 23 letter, Ortega suggested that the cardinal had failed to take the talks seriously.

Obando rejected these "insinuations" and told Ortega, "I'm sorry to inform you your letter does not reflect reality." Obando accused Ortega of ordering his delegation to press the cardinal to withdraw a proposal he presented in Guatemala.

Ortega came back the same day with another letter proposing a different agenda than the cardinal's. Finally, yesterday morning Obando penned an eight-page letter calling on the government to agree in the next round to set the dates when it would carry out reforms that Obando urged.

Obando's Guatemala proposal called on the government to allow full freedom of expression, free all political prisoners, resume a stalled dialogue with the opposition parties and revise its draft law. The contras were to gather their troops into zones for a 30-day truce.

In his final letter, however, Obando also dispelled rumors that he was about to quit by writing that he would continue to mediate even if the government ruled out his proposal.

{President Reagan, asked in Brussels whether he was concerned about the action taken by Ortega, said, "He concerns me by just being there," United Press International reported.

{State Department spokeswoman Phyllis Oakley said Ortega's call for direct negotiations without the presence of Obando was an attempt to undermine the peace process and a result of the failure of the House of Representatives to approve new military aid for the contras.}