SAPOA, Nicaragua--The government and Contra rebels declared a 60-day ceasefire late Wednesday, agreeing to negotiate an end to their six-year war and signing an accord that promises the rebels a role in Nicaragua's political process.

The agreement was the first concrete step toward ending the conflict, which has killed more than 40,000 people.

The ceasefire is to begin April 1, with negotiators to meet again April 6 in Managua to work out a permanent truce.

Daniel Ortega, president of the leftist Sandinista government, unexpectedly traveled to this small village on the Costa Rican border for the signing, which followed three days of talks.

He called the plan "a great challenge for all Nicaraguans" and called on the United States "to support this effort and get ready to normalize its relations with Nicaragua."

"I think, I hope and I believe that we have made a start, a firm start to end this war that is killing...sons of the same country, sons of the same mothers," said Contra leader Adolfo Calero.

Alfredo Cesar, another leader of the U.S.-supported rebels, said the nine points in the agreement "are not solely an opportunity to achieve peace and agreement, but the only opportunity."

The agreement, he said, was made "among Nicaraguans, full of good will and confidence in the future."

Neither the State Department nor the White House had any immediate comment on the agreement, which calls for the rebels to move into specified zones in Nicaragua during the first 15 days of April. Delegations from both sides will return to Sapoa on Monday to determine the zones.

The Contras also agreed to accept only humanitarian aid from a neutral organization. This would rule out further military aid from the United States, which President Reagan has tried in vain to secure from Congress. U.S. aid ended Feb. 29.

Even before the truce, Democratic leaders of the House of Representatives had said in Washington that they would act quickly to send food and clothing to the Contras if a ceasefire were worked out.

Under the agreement, the Sandinistas will release 100 political prisoners on Sunday and will free the remaining 1500 such prisoners at a date to be arranged at the April 6 talks.

The 1800 former members of President Anastasio Somoza's National Guard who have been in prison since the Sandinista revolution of July 1979 will be released after a final truce is worked out. Under Somoza, the army was called the National Guard.

The agreement says the Sandinista government will guarantee freedom of expression without restrictions, in accordance with a regional peace plan signed last August by Ortega and the presidents of four other Central American countries.

Many press liberties were barred in Nicaragua for almost six years under a state-of-emergency decree that has since been lifted.

The agreement says the Sandinista government will grant gradual amnesty and allow all political exiles to return to the country without fear of persecution and be able to participate in the political process.

The government also agreed to discuss the issue of military service in a national dialogue with the internal political opposition. The Contras had sought a suspension of conscription.

In Managua, opposition leaders hailed the agreement.

Eric Ramirez, president of the Social Christian Party, said, "It is a positive effort. Hopefully, it will contribute to ending the war and to the rise of the democratization of Nicaragua."

Gustavo Tablada of the Socialist Party called the move "a historic step...a serious and responsible compromise."