The Reagan administration believes it has "effectively blocked" what it views as an "unsatisfactory" regional peace settlement in Central America, according to a secret background paper prepared for a National Security Council meeting last Tuesday that the president attended.

The paper also outlines a wide-ranging plan to convince Americans and the rest of the world that Sunday's Nicaraguan elections were a "sham," promoting this view through U.S. embassies, politicians, labor organizations, non-government experts, and public reports.

The briefing paper, marked "secret/sensitive," was obtained by The Washington Post from governmental sources. It provides a detailed look at the administration's approach to the Sandinista government just days before elections in Nicaragua and the United States.

It is not known whether all the items in the briefing paper were discussed. Secretary of State George P. Shultz and Assistant Secretary for Inter-America Affairs Langhorne A. Motley also attended what was described by officials yesterday as a "briefing."

The paper discussed the administration's approach to the draft version of the Contadora peace treaty that was completed Sept. 7. It was negotiated by the foreign ministers of Mexico, Panama, Colombia and Venezuela, who first met for the purpose in 1982 on the small Panamanian island of Contadora.

The treaty's principal thrust is to reduce foreign military influence, establish mechanisms for arms control, and prevent the Central American countries from making or sponsoring war on each other.

On Sept. 21, Nicaragua unexpectedly announced it would sign the 55-page draft treaty. The Reagan administration had not publicly criticized it up to that point.

Since the Sandinistas announced their willingness to sign it, three countries -- Honduras, El Salvador and Costa Rica -- reversed their previous position of support for the treaty and, along with the United States, sought extensive modifications in the draft to improve verification and execution mechanisms.

The paper declares: "We have effectively blocked Contadora Group efforts to impose a second draft of a revised Contadora Act. Following intensive U.S. consultations with El Salvador, Honduras and Costa Rica, the Central American sic submitted a counterdraft to the Contadora states on Oct. 20, 1984 . . . that shifts concern within Contadora to a document broadly consistent with U.S. interests."

The United States repeatedly has portrayed the decision by Central American countries not to approve the initial draft treaty as one made independently by those countries, despite consultations.

The briefing paper expresses concern that a fourth Central American country, Guatemala, has been reluctant to back its three neighbors in seeking changes in the treaty. "We will continue to exert strong pressure on Guatemala to support the basic Core Four position," the paper says. The "uncertain support" of Guatemala is "a continuing problem," it adds. The term "Core Four" refers to Guatemala, Honduras, El Salvador and Costa Rica.

Mexico has been the most insistent promoter of signing the Sept. 7 version of the Contadora treaty. The briefing paper notes that Guatemala, because of its problems with guerrilla insurgency along the Mexican border, is seeking closer ties to Mexico, providing a "strong incentive" for Guatemala to lean toward the Mexican view.

But the paper concludes in a summary: "We have trumped the latest Nicaraguan/Mexican efforts to rush signature of an unsatisfactory Contadora agreement, and the initiative is now with the Core Four, although the situation remains fluid and requires careful management."

The paper notes that the administration recently has had "mixed" success in dealing with Nicaragua. "Congressional failure to fund the armed opposition is a serious loss, but our handling of the Nicaraguan election issue and Sandinista mistakes have shifted opinion against the sham elections," it says.

This was the administration line before and after the election. But the paper outlines ways in which this view should be promoted throughout the world.

It calls for encouraging "sympathetic American intellectuals and academics," "U.S. labor" and "selected U.S. political figures" to lobby their counterparts in Europe and Latin America, seeking critical statements about the election.

Another proposal was for the United States to use "selected embassies" in Europe and the Western Hemisphere to promote administration views.

"Embassy Bonn will approach West German ex-chancellor Willy Brandt to determine if he plans to make any public statements" on the election following the withdrawl of a key opposition party.

That withdrawal "has now left the Sandinistas holding a near worthless hand," the paper says.

The document also takes note of U.S.-Nicaraguan bilateral talks hosted by Mexico. At the 6th round, held idn September, the U.S. aide "tabled" a comprehensive statement by Nicaraqua, the background paper says, adding that the Sandinistas have adopted the Sept. 7 version of the Contadora treaty as their negotiating position vis-a-vis the United States as well.