In an atmosphere of urgency, eight Latin American foreign ministers sought in prolonged talks today to breathe new life into the Contadora peace process as an alternative to U.S. aid for anti-Sandinista guerrillas in Nicaragua.

But the two-day gathering here, the first in which all 13 Latin nations involved have come together, stumbled over sharp differences between Nicaragua's Marxist-led Sandinista government on the one hand and the U.S.-allied governments of El Salvador, Costa Rica and Honduras on the other. After struggling into the early morning hours with this fundamental split, the negotiators decided to hold an unscheduled third day of discussions.

The same disagreement has prevented substantial gains ever since Mexico, Panama, Colombia and Venezuela launched the effort on Contadora Island off Panama in January 1983 to prevent expanded warfare in the region. Vice President Rodolfo Castillo Claramount of El Salvador, who also is foreign minister, said that the discussions so far had proved fruitless and would resume Monday.

With the U.S. House of Representatives expected to reconsider a Reagan administration proposal for $100 million in mostly military aid for the rebels on April 15, the effort has taken on new immediacy in recent weeks, diplomats said. Against this background, the foreign ministers of Argentina, Peru, Brazil and Uruguay -- the so-called "support group" -- joined their four Contadora counterparts in intensified talks to usher the divided Central American nations into agreement on a formula for peace on the isthmus.

"There is a feeling of urgency that this meeting has to come out with something for public opinion indicating that we are taking positive steps," said a Latin diplomat involved in the talks.

Nicaraguan Foreign Minister Miguel d'Escoto insisted on obtaining a clear condemnation of the Reagan proposal for more aid to the rebels, called contras after the Spanish word for counterrevolutionaries, according to officials in the discussions. A Nicaraguan diplomat said the Sandinista government also maintained its earlier refusals to sign a Contadora treaty without a U.S. commitment to end support for the insurgency, which he said was the main obstacle to peace in Central America.

The Sandinista government cannot set aside the practical problem of a guerrilla war just to facilitate hopes for a regional treaty, a Nicaraguan diplomat said. "People are dying every day," he added.

Central American diplomatic sources said the Contadora ministers, with Foreign Minister Bernardo Sepulveda of Mexico playing a leading role, were seeking to soften the Nicaraguan stand in search of a compromise that would pledge signature of the treaty by a specific date later this year.

A Nicaraguan commitment to sign the treaty, even if indirect, was viewed as the key for U.S. congressmen and Latin American nations that consider the Contadora process to be a more promising course than the Reagan administration's approach of pressuring Nicaragua by financing the rebels' four-year-old guerrilla war there.

In more than three years of efforts, the Contadora negotiations have produced pledges to cooperate and seek agreement but no signatures on the treaty proposal, leading to predictions that the process may never be more than a forum for ritual talks.

The current draft treaty for which negotiators here are trying to find a formula for signature, provides for demilitarization of the area, the removal of foreign forces, guarantees for democratic government and protection of human rights.

Reflecting concern in Washington, three U.S. representatives -- Michael D. Barnes (D-Md.), James C. Slattery (D-Kan.) and William B. Richardson (D-N.M.) -- traveled to Panama for meetings with diplomats involved in the talks. Their aides said the main goal was to sound out Latin American officials on administration policy and gauge the potential of the Contadora talks to provide an alternative.

After meeting with the four Contadora foreign ministers, the congressmen said they were told by all four that their governments oppose the Reagan administration's proposal for aid to the insurgents.

Their comments were aimed at recent assertions by Reagan administration officials that Latin American governments support administration policy in private even though they may question it in public declarations.

"They were unanimous in their view," Barnes said. "They were strong in it."

Barnes said that the Contadora ministers requested that the House of Representatives delay any vote on the aid proposal for Nicaraguan insurgents to allow more time for the Contadora effort to produce a treaty signed by the five Central American nations.

"They asked us that we give Contadora a chance to work and that, therefore delay in the (House) vote would be a positive step," Richardson said. He added, "We think it's very critical that these meetings work because it may be the last chance for peace."

Officials of Honduras, Costa Rica and El Salvador said their ministers refused to participate in a condemnation of the Reagan administration's rebel aid plans despite the Nicaraguan demands.

The 13-nation meeting was called to encourage agreement on the long-prepared Contadora draft treaty, they said, not to comment on U.S.-Nicaraguan differences.

Five Central American nations -- Guatemala, Nicaragua, Honduras, Costa Rica and El Salvador -- joined in February in a declaration urging renewal of direct U.S.-Nicaraguan talks and removal of foreign interference from the region. Latin American diplomats said the Reagan administration expressed strong displeasure after that declaration, contending that those considerations fall outside the Contadora orbit.