GUATEMALA CITY, AUG. 7 -- The presidents of five Central American countries today signed a regional peace agreement that calls for cease-fires in the area's conflicts and democratic reforms that would require Nicaragua to open its internal political process.

The agreement, read at the end of a two-day Central American summit by Costa Rican President Oscar Arias Sanchez, calls for an amnesty for government opponents in each of the five countries, cease-fires in the region's wars, a series of "democratization" measures, a halt to all military aid for insurgents and renunciation by each country of the use of its territory for armed struggle against any of the others.

These measures are to take effect "simultaneously in a public form" 90 days from today's signing, Arias said.

The meeting of the presidents of Nicaragua, Guatemala, Honduras, El Salvador and Costa Rica signaled a breakthrough -- if the agreement is carried out -- toward ending the bloodshed in Nicaragua, El Salvador and Guatemala, the three Central American countries in which insurgents are battling the governments.

Costa Rica and Honduras, which do not have domestic insurgencies, border Nicaragua and are bound by parts of the agreement barring the use of one state's territory to attack another.

In Washington, the Reagan administration reacted cautiously. "We are anxiously awaiting details of this agreement," White House spokesman Roman Popaduik said. "After we have received it and have had a chance to analyze it, we will be in a position to comment."

Leaders of the Nicaraguan rebels, known as contras, expressed satisfaction with the agreement, but voiced interpretations of it that appeared likely to be debated.

The United States, which has backed the contras since 1981, is providing $100 million in aid to the effort. It also funds the Salvadoran government in its fight against Marxist rebels.

The signing ceremony in Guatemala's ornate National Palace, after two days of intense debate, seemed to produce optimism that peace in the war-ravaged area may be at hand.

"It's a long road to build peace," said Guatemalan President Vinicio Cerezo in closing the summit. "Today in Guatemala we can affirm that utopia is starting to become a reality."

President Arias said later that the agreement showed the world that "rationality can prevail" and that there is a "new will" for peace in Central America.

"I was able to persuade my colleagues that either we reach an agreement today or there will be an escalation of war, and no one wants that," Arias said. He said the leaders of Nicaragua and El Salvador had shown "flexibility" because "they are both quite desperate and both want peace." He said Nicaraguan President Daniel Ortega had "made a lot of concessions."

Contra leaders said the agreement allowed a cease-fire "in place" in Nicaragua that would permit their forces to receive "humanitarian" aid without requiring them to lay down their arms.

Arias said the cease-fire provision, which is not specific, has to be negotiated by the parties concerned under the auspices of a commission to be established soon.

In a written statement, the contras' six-member directorate noted that the agreement does not mention any "sanctions" for failure to comply with it, and said the Sandinistas had broken promises before. It added that the contras "have not renounced the right to defend the interests of the Nicaraguan people."

According to Alfredo Cesar, one of the contra leaders assembled here to observe the summit, the agreement means that "Daniel Ortega has for the first time recognized implicitly the legitimacy of our cause."

Sandinista leaders left Guatemala shortly after the summit's concluding ceremony and were not available for comment on the accord.

The agreement appeared to follow closely a proposal advanced by Arias in February. The major difference, Central American officials said, is that today's accord provides for the cease-fire, democratization and other measures to take effect simultaneously.

This appeared to meet at least one stated concern of the Reagan administration, which feared that the original Arias plan would disarm the contras and lift military pressure on the leftist Sandinista government before they carried out any democratic reforms. Today's plan, however, does not resolve the widely perceived underlying concern of the Reagan administration that a peace agreement would leave the Sandinistas in power.

It also did not mention halting foreign military aid to governments, an omission that would allow the Sandinistas to continue receiving substantial Soviet aid and El Salvador to keep getting U.S. assistance.

The accord calls for each government concerned to hold talks with its civilian political opponents and those covered by a general amnesty for antigovernment combatants. In conjunction with the amnesty, guerrilla groups would release all prisoners they hold.

To put cease-fires into effect, "commissions of national reconciliation" would be established in each country. The agreement calls on the governments to invite participation in the commissions within five days of the signing and for the composition of the commissions to be announced within 15 days after that.

In an "exhortation" for an end to hostilities, the accord says, "The governments of the states commit themselves to carry out all the actions necessary to achieve an effective cease-fire within the constitutional framework."

Contra leaders said afterward that this meant the Sandinistas would have to negotiate with them, something Managua previously has refused to do.

At the time that the cease-fires take effect, the agreement calls for implementation of "an authentic democratic, pluralistic and participative process" in which human rights and national sovereignty would be safeguarded. It says this should take place in a "verifiable manner" and without foreign interference.

Specifically, the agreement calls for complete freedom of the press, political freedoms for all groups and free and regular elections under international monitoring.

It calls for elections for a newly conceived "Central American Parliament" to be established next year, and for municipal, legislative and presidential elections in accordance with each country's constitution. All these elections would be monitored by observers from international organizations such as the United Nations and Organization of American States.

The accord calls for the cessation of all "military, logistical, financial {and} propagandistic aid" to insurgencies, as well as "manpower, arms, munitions and equipment" as "an essential element" to stop Central America's wars. It allows aid for "repatriation" or relocation or for combatants' "reintegration into normal life."

It says that an "international verification commission" consisting of the U.N. and OAS secretary generals and eight Latin American foreign ministers would ensure that the agreements are carried out. In 15 days, the accord says, the foreign ministers of the five Central American states are to meet to carry the agreement forward so its provisions can take effect in 90 days.

The international commission would then meet in 120 days from today's signing to assess progress in implementing the agreement and prepare a report for the five presidents, who would meet against 150 days from today.

The signers, besides Arias, Cerezo and Ortega, were Honduran President Jose Azcona and Salvadoran President Jose Napoleon Duarte.