U.S. sponsored mediation of the Nicaraguan crisis has encountered a major setback that resulted yesterday in the urgent recall to Washington of the chief U.S. mediator in Managua, the U.S. ambassador there, and the head of the U.S. Army Southern Command in Panama.

The temporary recall of the three followed President Anastasio Somoza's refusal to accept a "final" mediation proposal for a nationwife plebiscite to determine if he should continue in office.

In its harshest and most pointed criticism of Somoza since the mediation began nearly three months ago, the State Department yesterday described Somoza's refusal to accept the "fair and workable" mediators' proposal as a "serious snag."

A department release noted that the coalition Broad Opposition Front seeking Somoza's ouster "has fully accepted the plan."

Along with Ambassador William G. Bowdler, who heads the U.S. delegation to an international mediating team including representatives of the Dominican Republic and Guatemala, the recalls included U.S. Ambassador to Nicaragua Mauricio Solaun and the Southern Command chief, Lt. Gen. Dennis C. McAuliffe.

McAuliffe had met with Somoza in Nicaragua last Thursday in an effort to convince him of the Pentagon's concern for a peaceful resolution of the crisis, which led to three weeks of civil war last September between Somoza's National Guard troops and civilians led by Sandinista Liberation Front guerrillas.

Bowdler is expected to return to Managua to meet with Somoza today. But Somoza's rejection of the "final" plan appeared to leave the mediators with few, if any, new options to keep alive their attempts to avoid further violence.

The final plan was presented to Somoza and the opposition Thursday night, with a 24-hour deadline. A variation on a plan previously accepted, with conditions, by both sides, it included Somoza's agreement to immediately resign and go into voluntary exile for two years should he lose a referendum tentatively scheduled for Feb. 25.

The plan also stipulated that both Somoza's son, National Guard Maj. Anastasio Somoza Portocarrero, and Somoza's half-brother, National Guard Inspector General Jose Somoza, leave the country immediately.

Although both of these stipulations had been rejected by Somoza in previous mediation, his rejection of the final plan was based on a provision that called for the referendum to be organized, supervised and fully controlled by an "international organization," presumably the Organization of American States.

While the Board Oposition Front, a coalition of more than a dozen antiSomoza political and civic groups, accepted all provisions, Somoza, in a letter to the mediators, described the referendum plans as "interventionist."

Somoza said in the letter that he would present his own "counterproposal" to the plan, but sources close to the mediating team said they were not interested.

There has been a series of rejected proposals and counterproposals by Somoza and the Broad Front throughout the mediating process, usually hinged on the question of whether Somoza would leave the country following his loss, or if the opposition would agree to join a coalition government with him if he won.

Two weeks ago, representatives of Somoza's Liberal Party and opposition delegates met for the first time face to face since the mediation began. The talks quickly broke down over the issue of a coalition government, which the opposition rejected in the belief that it would discredit itself in the eyes of the massively anti-Somoza population.

The opposition also was reluctant to participate, despite pressure from the mediators, in a referndum controlled by the Somoza government based on a fear that fraudulent vote counting would result in Somoza being declared the winner.

Both of those questions appeared to be answered in the final mediation plan last Thursday. Although Somoza's rejection came within the 24-hour deadline, the mediators apparently spent the weekend trying to persuade him to change his mind, giving up only yesterday.

Meanwhile, the Nicaraguan government yesterday announced that it had closed its southern border with Costa Rica because of alleged Sandinista attacks launched from the neighboring country.

Although the two nations broke relations last month, the border had remained open and yesterday's decision could have serious consequences for Central American trade and transportation conducted via the Pan American highway that is the main road link from Mexico to Panama.

Worsening relations between the two is also likely to heighten U.S. and Latin American concern that the Nicaraguan crisis will become more internationalized.

The United States believes that both Venezuela and Panama, whose governments are staunchly anti-Somoza, are aiding the Sandinistas in their preparations for another all-out attack on Nicaragua from bases allegedly located in Costa Rica. At the same time, the governments of El Salvador and Guatemala, and to a lesser extent Honduras, are considered sympathetic to Somoza and available to aid him under the terms of a Central American defense pact.

In recent weeks, U.S. officials have attempted to convey to other Latin American governments their strong disapproval of aid to the guerrillas and have tried to convince them that further arming of the Sandinistas could sabotage the mediating talks.

Evidence that the talks have conclusively failed could remove one of the few remaining impediments to a resumption of last September's civil war, in the view of many Latin American leaders.