Nicaragua's opposition coalition has presented President Anastasio Somoza with a proposal calling for his immediate resignation and departure from the country, to be followed by a series of transitional steps leading to the election of a new president by Sept. 1981.

Should Somoza refuse to resign, the proposal calls for the Nicaraguan Congress to declare him incapacitated and remove him from office.

The plan, offered by the Broad Opposition Front coalition of political, business, civic and labor groups, is the first concrete step taken thus far by either side since U.S. organized mediation of the Nicaraguan conflict began in Managua three weeks ago.

The proposal was presented last Wednesday to the mediating team formed by representatives of the United States, the Dominican Republic and Guatemale. The mediators officially handed it over to Somoza Thursday, without public comment tr for add one.

At a news conference yesterday, Somoza said he would not comment on the plan, which was published in a small circulation local newspaper, until next week so that his National Liberal party and the citizenry could study it. Earlier, in an interview with UPI immediately after receiving the proposal Thursday, Somaza called it an "idealistic proposition."

Asked in the interview if he would resign. Somoza said "no."

That "no" has been Somoza's answer ever since the resignation demands were first made early this year, and throughtout a civil war that tore Nicaragua apart last month. Somoza has repeatedly stated that his resignation before the 1981 expiration of his term would be a violation of the Nicaraguan constitution.

The opposition plan is a complicated series of transitions that attempts to override Somoza's argument by calling for the immediate election of a new president from within the Congress, as spelled out in the constitution in the event of resignation or incapacity of a president. The new president, who would likely be a member of Somoza's Liberal Party since it currently holds a legislative majority, would quickly be replaced by a "council of state."

The council, composed of two Liberal Party members and two members of the Broad Opposition Front, would then select a "provisional government junta" of three civilians to govern the country until elections in 1981.

The "legal installation of a civilian junta would require a constitutional amendment, and such amendments can be implemented only by vote of two separate legislative sessions. Constitutional lawyers are now trying to figure out if the current session ends with the end of the calendar year, or on April 30. Such a judgment presumably would determine when the stages of the plan could be implemented.

The plan is a compromise between differing schools of thought within the loosely allied Opposition Front itself and, while it is unlikely to meet with Somaza's approval, it already appears to have caused at least one defection from the opposition ranks.

On Wednesday, the day the opposition turned over its plan to the mediating team, members of the "Group of Twelve," one of the most radical elements of the Opposition Front and considered the group most closely allied with the Sandinista guerrillas, resigned from the negotiations and took asylum in the Mexican Embassy in Managua.

Apparently referring to the inclusion of Liberal Party members in the plan, "the Twelve" charged that the United States had pressed a compromise on the opposition that would "leave the corrupt structures of the Somoza framework still intact."

The spectre of what "the Twelve" called U.S. "interventionism" has hung over the negotiations since they began. While they declined comment on the allegations, it is known that U.S. officials have felt the mediation was being slowed by the inability of the opposition to agree on a plan that the mediators considered both feasible and conceivably acceptable to Somoza.

The withdrawal by "the Twelve led an increasingly jumpy Managua to speculate that a new Sandinista offensive is planned for the near future.

Sandinista guerrilla attacks in early September sparked the civil war, and while the guerrillas have tacitly respected the negotiations by refraining from new large-scale assaults since they began, they are unlikely to accept any plan that includes Liberal Party participation or the continued existence of the National Guard.

One report from Managua described the atmosphere as "surreal." As occasional gunbattles shattered the nighttime curfew calm and National Guard roadblocks sent traffic into a maze of sidestreets, the mediators moved between meetings with the opposition, held at the offices of the Archbishop of Managua, and sessions with Somoza.

The mediators have now met at least seven times with the president, who, until he appointed several Liberal Party attorneys as representatives yesterday, has done his negotiating in person. The meetings take place in his office, called "the bunker," located inside a National Guard compound in the capital. Soldiers last month built a concrete wall outside the office, and this week began construction of another wall around the fenced compound itself.

While the opposition seems to grow more tense as the negotiations drag on, Somoza, at least in his public appearances, has appeared calm and largely unconcerned with his problems.