The killing of one of Nicaragua's most prominent business leaders by government security agents, amid allegations that he was involved in a massive plot to overthrow Sandinista rule, has provoked the most severe crisis here since the revolution 16 months ago.

Talk of cooperation between the private sector and the leftist government -- vitally needed to restore the battered economy -- has given way to bitterly divisive charges of political crimes and coup attempts, provoking a level of tension and mistrust that many Nicaraguans had hoped was gone with the fall of dictator Anastasio Somoza.

At a press conference today, the sandinista chief of state security, Lenin Cerna, attempted to clear up some of the circumstances surrounding the death last Tuesday of Jorge Salazar, 41, vice president of the influential Superior Council of Private Enterprise.

Cerna presented three alleged conspirators, who confessed to working with Salazar in an attempt to obtain arms and enlist the cooperation of elements inside and outside the country to launch a coup this month.

One of the three, Leonardo Somarriba, himself an influential businessman, said he and Salazar arranged meetings with some of Somoza's former commanders as well as with the foreign minister of El Salvador, Fidel Chavez Mena, and Salvadoran junta member Jaime Abdul Gutierrez in unsuccessful attempts to generate support for the coup.

The attempt would have included an invasion by pro-Somoza exiles and an internal uprising, Cerna said. But he did not explain the exact circumstances in which Salazar was killed -- an act that private sector leaders here have said "shows all the characteristics of a political crime."

According to the official version of Salazar's death, he was meeting with Ernesto Montada at a gas station outside of Managua when the two of them were surprised by Sandinista security agents.

Montada allegedly fired a pistol. The security agent also fired. Montada was captured unhurt. Salazar, described as unarmed, was shot seven times. Only later were six automatic rifles allegedly discovered in the back of his car.

Montada, who was not brought forward at today's press conference, was an official in the immigration section of the Ministry of Interior, according to Cerna. That ministry, under Maj. Tomas Borge, also controls state security.

It was by mere accident that Salazar was killed and Montada lived, according to Cerna and other officials.

Borge told reporters tonight, "It would have been very easy to make up a story. We could have put a gun in his hand . . . . We are not lying."

But few of Salazar's friends and acquaintances accept the official account.

"Whether Jorge was involved in a plot or not," said one associate of Salazar's, "you don't kill a guy who was unarmed. And if Jorge Salazar was involved it shows the desperation of people like him."

Desperation has indeed been growing among former supporters of the Sandinistas -- especially the private sector -- for several months. The economy has failed to reach the projected levels. Production is off. And political freedoms are rapidly diminishing.

Since the Sandinista guerrilla-led revolutionaries celebrated their first year of the revolution on July 19 with a massive rally and a display of military organization, Nicaragua has taken several steps to consolidate power while increasingly restricting the opposition.

It had been expected that on July 19 the Sandinistas would announce a fixed date for an election. They did not.

Instead, on Aug. 23 it was announced that elections would not be held until 1985 and that no campaigning would be allowed until 1984. The economic situation was given as the reason for the delay.

On Sept. 17, laws were instituted prohibiting the local press from printing any material related to state security or economic problems without prior government approval.

On Nov. 9, opposition leader Alfonso Robelo, a former member of the five-person revolutionary junta, was forbidden to hold a rally at the town of Nanbaime. Shortly thereafter, a Sandinista youth group sacked the offices of Robelo's Nicaraguan Democratic Movement in Managua.

Representatives of private enterprise and the opposition parties then withdrew the opposition parties then withdrew from the quasi-parliamentary Council of State.

On Nov. 17, U.S. Ambassador Lawrence Pezzullo said, "It's a much more brittle Nicaragua than it was six months ago. The level of compromise is lower. The situation could easily degenerate and just get more and more fractionalized."

In the far more cooperative atmosphere of last April, the Sandinistas and the private sector, which still control 60 percent of the nation's productive capacity, rapidly reached a series of agreements. Most, with the important exceptions of elections and complete press freedom, were kept.

Now, said one of the businessmen who negotiated the April compromise, "we are in a deadlock."

Arturo fCruz, the middle-of-the-road non-Sandinista who replaced Robelo on the junta, said yesterday that there is indeed a problem of credibility on both sides now.

"It's true that the government has hardened its position," said Cruz, "but mostly in response to the private sector's attitude. Sometimes the dissenters create a situation where the revolutionary leadership might feel -- I hate to use the word -- blackmailed."

No one here sees an immediate solution to the crisis and no one seems certain of where it will lead. As newspaper editor Pedro J. Chamorro Jr., the object of many government attacks because of his paper's outspoken opposition to the Sandinista leadership and some of its policies, said, "1981 will be a year of crisis."