MANAGUA, NICARAGUA, SEPT. 30 -- With five weeks to go before the deadline of the Central American peace plan, opponents of Nicaragua's ruling Sandinistas are cautiously promoting and testing it.

The discussions sparked by the plan, which calls for full political freedom in all five Central American countries, are the liveliest held here in three years, Sandinista officials, opposition leaders and other residents agree.

It remains far from certain, however, whether the plan will bring peace, or even a lasting increase in political freedom, to Nicaragua.

"Nicaraguans should not lean toward passive defeatism; we should do everything we can to see the terms of the accord are met," exhorted Nicaragua's Roman Catholic bishops in a Sept. 17 pastoral letter. The plan, signed in Guatemala Aug. 7, also calls for a cease-fire, talks between governments and unarmed opposition groups in each country, and an end to outside aid to insurgencies.

"We greeted this accord with joy and with doubts. Today we still have more doubts than joy," said Carlos Huembes, president of a coalition of opposition labor and business groups and political parties, at a press conference yesterday.

Nicaragua has done more than any other Central American nation to move toward full compliance by Nov. 7. It gave permission for the opposition daily La Prensa and the Catholic Radio to reopen, ended censorhip, named the conservative Cardinal Miguel Obando y Bravo to head a national reconciliation commission and announced that its troops will observe localized cease-fires to facilitate the commission's work.

But for the opposition, cowed by years of harassment, the going is rocky. Last Thursday, Erick Ramirez, president of the opposition Social Christian Party, was allowed into Managua's Model Jail to visit eight of 20 Social Christian political prisoners. But Ramirez said Sandinista authorities informed him the other 12 were no longer in that jail, then refused to tell alarmed relatives where the prisoners were.

On Sunday, 3,000 sympathizers of the Social Christian Party marched noisily through the streets of Managua to celebrate the party's 30th anniversary. The government did not interfere during the demonstration. But 18 Social Christian men were detained Sunday night and hastily drafted into the Sandinista Popular Army as they returned to their rural homes, Ramirez said.

Thursday night, Ramirez declined an invitation to join in a live radio debate about the peace plan with a Sandinista official, the first of its kind since a national election campaign in late 1984. Based on past experience, Ramirez said, he feared the two government-controlled dailies would twist his comments in their reports of the broadcast.

Sunday's march was the party's largest since the Sandinistas came to power in a 1979 revolution, Ramirez said. It showed that some sympathizers are willing to take to the streets again after public rallies had been banned for three years under a state of emergency, but many are not. The Sandinista party easily turned out a crowd of 8,000 for a march yesterday in support of the peace plan.

President Daniel Ortega and other Sandinista leaders went forward with the peace measures only after debating with many party militants who feared that their socialist revolutionary programs might be sacrificed, Sandinista officials said.

The Sandinista National Liberation Front's chief ideologue, Bayardo Arce, seeking to reassure party stalwarts in a recent speech, warned the opposition that the party would keep control over any peace-plan initiatives.

"You can forget the idea that because you ask us to do something, the government will do it," Arce told the opposition. "Nothing we do under the peace plan should be seen as a sign of our weakness. So we're warning you right now, we won't respond to any demands."

On the sensitive point of the cease-fire, top Sandinista leaders said they will implement it unilaterally, rejecting direct talks with leaders of the Nicaraguan Resistance, the alliance of the U.S.-backed rebels fighting to overthrow the government.

The Sandinistas have consistently refused to meet with rebel leaders, saying they will only allow discussions with rebel field commanders inside Nicaragua through intermediaries from the National Reconciliation Commission. This would be to discuss an amnesty for rebels who lay down their arms. The accord only requires talks between governments and their unarmed opposition.

Pressure is mounting, however, for direct talks with the rebels, known as counterrevolutionaries or contras. Nicaragua's bishops argued that a peace settlement without the contras cannot last long. Costa Rican President Oscar Arias asked Ortega to allow Obando to mediate between the government and the contras, and the other Central American nations have seconded Arias' call.

Last week, Obando won a crucial round of tactical infighting with the Sandinistas. After a three-hour meeting with Ortega, Obando announced the reshaping of the local commissions that will extend the work of the National Reconciliation Commission into the embattled countryside. The local commissions will be under the control of Obando's national commission and also of the bishops in each province.

However, Obando decided not to risk his prestige by throwing himself into the commission's day-to-day work, the cardinal told a senior European diplomat. To the dismay of the opposition, the cardinal left Sunday for a month-long trip to Rome. In Obando's absence, the commission will be run by Sandinista Vice President Sergio Ramirez.

Despite the plan's difficulties in Nicaragua, the Sandinistas' relations with their regional neighbors have improved as a result of the Reagan administration's negative assessment of the peace plan, according to Central American diplomats here and in Washington. A regional consensus is emerging in favor of bilateral talks between Managua and Washington. The Reagan administration insists the Sandinistas must talk with the contras first.