Negotiations between the ruling, leftist Sandinista Front and the largely conservative opposition coalition have broken down again, feeding concern that the differences between the two sides are too profound to reconcile and that the level of violence in Nicaragua will rise.

An agreement that might have brought a cease-fire in the 30-month-old guerrilla war and broadened impending elections was being negotiated this week at a meeting of the Socialist International in Rio de Janeiro, Brazil. But the talks between Bayardo Arce of the nine-man Sandinista National Directorate, and Arturo Cruz, presidential candidate of the opposition Democratic Coordinator coalition, collapsed.

Today, Coordinator officials said former chancellor Willy Brandt of West Germany, representing the Socialist International, had written a letter to Nicaraguan head of state Daniel Ortega, asking that the talks be resumed. Ortega is in the United States.

Officials of the Socialist International and other international leaders have been pressuring the Sandinistas to negotiate their differences with Cruz and both sides seek to maintain close ties with that association of social democratic parties.

But the failure of the latest negotiations was no surprise for many observers here, given the deep ideological differences between the government and the Coordinator.

"This isn't like the United States, where the Democrats and Republicans believe in the same basic system of government," said opposition leader Mario Rappaccioli. "Here we are talking about two completely different systems. They are totalitarian and we are democrats. It won't work."

Leaders of the Coordinator, which represents four centrist and conservative political parties, two labor unions and an association of businessmen, insists that the Sandinistas want to turn Nicaragua into a Marxist state, eliminating private enterprise and any opposition.

The Sandinistas in turn charge that the Coordinator is part of a Reagan administration plan to overthrow their government. Leaders of both sides have fanned the flames of the conflict in recent days.

Last Friday, in a highly charged speech before tens of thousands of supporters in Managua, Ortega, who is also the Sandinista presidential candidate, warned opposition leaders that if they continued to call attacks by U.S.-funded rebels "a civil war," they just might end up with such a war on their hands.

"And then whole classes of people will disappear," warned Ortega. "The people will obliterate some classes and then those classes of people will know the fury of the public."

Meanwhile, a centrist opposition figure, who asked not to be identified, complained to a journalist about some of his opposition colleagues. "The far right in this country is impossible," he said, referring to some members of the Coordinator. "All they talk about is preparing the way for a U.S. invasion. They won't agree to anything."

The fact that the leaderships of both sides are divided into factions is thought to have made negotiations between them more difficult.

While Cruz is a former member of the current government and is widely described as a moderate, the ultraconservative businessmen who represent the Supreme Council of Private Enterprise in the Coordinator are said to have heavy influence on his coalition. It is believed by many here that if Cruz were to run, independent ranchers and planters and other business organizations would be his base.

On the Sandinista side, diplomats and opposition politicians say a "hard-line" faction within the nine-man National Directorate is little inclined to make concessions to the Coordinator. Those most often mentioned as being hard-liners are Arce, Interior Minister Tomas Borge and Planning Minister Henri Ruiz.

But both sides had reason to want an agreement. "Cruz and his people have to be concerned that if they don't participate in elections, they will end up off the political map here altogether," said a diplomat.

In the case of the Sandinistas, attacks by U.S.-funded rebels have claimed more than 7,000 lives and have caused some $250 million in damages, say the ruling movement's leaders.

Ortega announced earlier this year that the Sandinistas would spend 25 percent of the 1984 national budget on defense, draining a crippled economy of investment capital. An obligatory military service law has caused dissent, leading many families to leave the country and provoking sporadic demonstrations against the government.

But instead of an agreement, both sides are left talking about the possibility of a larger war. Cruz has expressed the fear that if the Sandinistas do not allow a "democratic opening here, all Central America will fall on Nicaragua." He has said he believes that the United States could intervene here.

Ortega in his U.N. speech yesterday, said the rebels were planning a new offensive designed to disrupt the Nov. 4 elections and the United States was preparing to intervene.

Jackson Diehl of The Washington Post Foreign Service reported from Brasilia:

Arce said at a press conference here today that he had arrived at a "consensus" with members of the opposition front during the talks in Rio, but the talks broke down after the opposition delegation informed him it did not have the authority to formalize an agreement.

Arce said that under the preliminary agreement, the opposition front had agreed to register its candidates on Monday, which was the deadline. They also agreed that the anti-Sandinista guerrilla fronts would announce a cease-fire by Oct. 10 and would cease military activities by Oct. 25.

The Sandinistas promised that when the military activitity ceased, they would postpone the elections until mid-January, he said.

In addition to the Democratic Coordinator's failure to confirm the agreement, Arce said, the opposition reported back that they could not guarantee that all military activity would cease by Oct 25. They asked that the Sandinistas agreed to postpone the elections even if the fighting had not stopped by then, Arce said. "We cannot accept that."

He said there was little chance an agreement could now be reached with the opposition but he added that if the opposition leaders "arrive back in Managua and are able to arrive at a unified position on the agreements and make us a proposal, we will have to consider it."

In Rio, Adan Fletes, the Coordinator's would-be vice presidential candidate, disputed the account of Arce and said his pullout "was really inexplicable." Fletes said his group was "open for new talks at any time" but it was up to the Sandinistas to take the initiative.