SAN JOSE, COSTA RICA, JAN. 16 -- The presidents of five Central American countries agreed today to take steps immediately to comply with a regional peace plan that they signed last year, and Nicaragua announced new concessions designed to head off U.S. aid to anti-Sandinista rebels.

A statement by Nicaraguan President Daniel Ortega before the summit meeting's final communique was released said his government would suspend a state of emergency in the entire country starting today, immediately "convoke direct talks" with the rebels on a cease-fire, implement a partial amnesty once a cease-fire is agreed and hold elections for seats in the Central American parliament and municipal offices.

Ortega's statement did not specifically call for a cessation of U.S. aid to the rebels known as contras, nor did the final communique issued by the five presidents. But such a step is one of the provisions of the peace plan signed by the five presidents in Guatemala Aug. 7.

The announcement of these measures, including the first face-to-face talks between representatives of the Sandinista government and the rebels, came shortly after the Nicaraguan Interior Ministry announced the arrest of four opposition politicians in Managua on charges of involvement in a "terrorist conspiracy." {Details on Page A28}

A conference participant close to the Nicaraguan delegation said Ortega's concessions went beyond what had been expected to come out of the summit meeting.

At a press conference after the communique was issued, Ortega said, "The U.S. Congress should respect the will of the Central American presidents. They should not approve one more cent for the mercenary forces. Each dollar they approve would be a dollar to kill the Central American accords that we have reaffirmed today."

Congress is to vote early next month on a Reagan administration proposal for new funds for the contras.

In his statement, Ortega justified the arrest of the opposition figures as a legitimate response to a "conspiracy" to form "an internal front with the contras."

"This is not civic opposition," Ortega said. "This cannot be tolerated."

The four were picked up for having met with contra leaders this week in Guatemala, where they agreed to ask Ortega to participate in a "tripartite dialogue" with them.

Sources close to Ortega later charged that the opposition figures' trip to Guatemala had been organized by the U.S. State Department to form an alliance with the armed rebellion.

One source said, "President Ortega feels it's a ploy of the State Department to distract attention from the summit."

Ortega indicated in the news conference that if "external aggression" continued, the state of emergency could be reimposed. He said it remained to be decided if the lifting of the five-year-old emergency, which suspends a number of civil rights, would include the abolition of special political courts called People's Anti-Somocista Tribunals."

Last night Ortega met with U.S. Democratic congressmen led by Lee Hamilton of Indiana, who told him that unless he made major concessions there was no hope of stopping new aid to the contras.

Ortega's statement said the cease-fire talks would take place "within the framework of the Esquipulas II accords," a formulation that does not contemplate discussions of political changes that the rebels have demanded. Esquipulas II refers to the Central American peace accord signed last August in Guatemala by Ortega and the presidents of El Salvador, Honduras, Costa Rica and Guatemala.

The summit meeting here was called for under that agreement to evaluate compliance with the peace accord.

Costa Rican President Oscar Arias, who won the Nobel Peace Prize for drafting and promoting the peace plan, said after today's communique was signed that the five presidents had not been completely satisfied with compliance thus far so they now were committing themselves to "total compliance, without excuses."

The other presidents attending the meeting were Vinicio Cerezo of Guatemala, Jose Napoleon Duarte of El Salvador and Jose Azcona of Honduras.

Ortega's statement said his government would include Nicaraguans on the team that would negotiate the cease-fire with the mediation of Nicaragua's Roman Catholic primate, Cardinal Miguel Obando y Bravo. At his press conference, Ortega said another Nicaraguan would join an American and a West German in the face-to-face negotiations.

In late December indirect cease-fire talks in the Dominican Republic were stalled when Managua sent a negotiating team made up of foreigners which the contras rejected.

The statement said there would be a meeting "immediately" in the Costa Rican capital.

It also said Nicaragua would implement its Amnesty Law No. 33 "immediately upon the achievement of an effective cease-fire agreement and the incorporation of the armed groups into the civil sector."

That amnesty law does not cover more than 2,000 former National Guardsmen who have been held since the Sandinistas took power in 1979. How many other prisoners it would cover is in dispute.

The statement also said that if no cease-fire agreement is reached, the Nicaraguan government would free the eligible prisoners if the U.S. government agreed to accept them. It said they could "return to Nicaragua as soon as the war ends."

The statement pledged to hold the municipal elections "in the manner established by the constitution," but did not set a time frame. The announcement came after an extra session of a turbulent summit meeting, which originally had been scheduled to begin and end yesterday.

Salvadoran officials said the five presidents had accepted El Salvador's proposal to reject any extension of the peace accord's compliance deadline, which expired earlier this month. The peace plan originally called for cease-fires, amnesties, democratization, cessation of outside aid to rebel groups and an end to the use of one state's territory to attack another -- all to take effect simultaneously Nov. 5 last year.

The summit meeting's final communique, read by Costa Rican President Arias, said, "Because the commitments of Esquipulas II were not completely complied with, {the presidents} commit themselves to unconditional and unilateral obligations that require of the governments a total compliance without excuses. Among these are dialogue, cease-fire talks, general amnesty and, above all, democratization."

It said the latter included lifting states of emergency, "total freedom of the press," political pluralism and an end to special courts. "The stated commitments that the governments have not fulfilled must be fulfilled immediately and in a public and evident way," the statement said.