Efforts by Western Europeans and Latin Americans to seek a negotiated solution of El Salvador's civil war have been stymied by the refusal, first by the Salvadoran opposition and now by the ruling junta and its backers in the United States, to sit down at the mediation table.

Top political and government leaders from Mexico, Costa Rica, Panama, Venezuela, West Germany and Canada, among others, have been engaged in an almost constant regional shuttle over the past several months among the principals involved in the war in attempts to end the fighting which has taken, by some estimates, as many as 20,000 lives in the past 17 months.

A new effort to bring the sides together is being made this week by Edward Broadbent, leader of Canada's New Democratic Party. After a whirlwind tour of the region, including visits to Mexico, El Salvador and Cuba, Broadbent is expected in Washington Thursday, where he will hold talks with Assistant Secretary of State-designate Thomas O. Enders.

Broadbent's initiative already has been rejected by the Salvadoran government.

Diplomats in the region have begun accusing the Reagan administration of blocking their efforts, and increasingly feel that no political negotiation is possible unless Washington changes its current policy of backing a military victory over leftist guerrillas seeking to overthrow the Salvadoran government, followed by a limited election.

"The United States is insincere," former Venezuelan president Carlos Andres Perez, a social democrat, charged in a recent interview in Caracas. "The start of negotiations depends entirely on [the Americans], but they are trying to cover up that fact."

Now an opposition leader and long an influential figure on the Latin American political scene, Perez said the "call for elections to solve El Salvador's problems is madness and stupidity. It has no chance. Even if the guerrillas do not have enough strength to overthrow the government, they have enough power and popular support to prevent the elections from being valid."

Initiatives for a political dialogue to stop the war have been ongoing since all-out hostilities began in El Salvador early last year. The Carter administration, which backed the Salvadoran civilian-military junta with economic aid but only limited military assistance, held private conversations with the Democratic Revolutionary Front (FDR), the political arm of El Salvador's leftist guerrilla movement. But the Carter team expressed little sustained interest in bringing together the FDR and the junta.

Last fall, the Catholic Church in El Salvador offered to mediate, an offer that was refused by the opposition. The FDR then countered that it would speak directly only to the United States, which it considers the principal source of the junta's power, rather than to the junta itself, a coupinstalled government led by appointed President Jose Napoleon Duarte.

The FDR now insists that it is willing to talk to the junta itself via outside mediators.

In recent months, following a shortlived guerrilla offensive in January and the subsequent large increase in direct U.S. military aid to the government by the Reagan administration, the junta, with U.S. support, has rejected all outside efforts to negotiate a political solution with the left that would end the fighting.

Although numerous reports from El Salvador indicate that the infusion of U.S. military hardware and advisers has failed to dramatically tip the balance in favor of the government forces, both the junta and the United States, according to officials of both governments, now believe that a military victory over the left, which would eliminate the need to negotiate political power, is at hand.

At the same time, a key State Department official said that negotiations might undermine civilians within the junta and, even more dangerously, split the armed forces, with the most hard-line sector gaining the upper hand.

Both the U.S. and Salvadoran governments now have placed their emphasis on legislative elections scheduled for March 1982 and presidential elections scheduled for 1983, and have said that if opposition political parties in the FDR want to participate in a new political order, they can campaign for office.

Numerous political leaders in Latin America and Western Europe, however, along with the FDR, have stressed their belief that without a prior understanding between the government and the opposition, an election in the middle of a civil war would be futile.

Rather than improving the political climate, advocates of peace talks point out, the Salvadoran armed forces last month published a list of 137 "terrorists," which included even the more moderate leaders of the opposition.

"Election with death lists of the opposition circulating, in the presence of U.S. military, is macabre. You might as well hold them in the cemeteries," said Pierre Schori, international secretary of Sweden's Social Democrat Party and one of the many Europeans visiting here who have involved themselves in the Salvadoran crisis.

Largely in response to the U.S. military aid, and fearing greater U.S. involvement, the Socialist International -- an anticommunist political organization of social democratic, socialist and labor parties from 39 countries -- and nations such as Mexico, Nicaragua, Cuba and Panama, which sympathize with the FDR, have taken the lead to search for some common ground between it and the junta.

Western European members of the Socialist International believe that their chance to pressure Washington to rethink its policy has improved since the inauguration of Socialist Francois Mitterrand as president of France. Mitterrand's foreign minister, Claude Cheysson, is scheduled for talks in Washington with Secretary of State Alexander M. Haig Jr. on Friday.

Efforts to negotiate a settlement have been hampered by suspicion on both sides of the Salvadoran conflict that the other wants to bring it to the mediation table simply to gain time to consolidate its military position.

Last April, Socialist International emissary Hans Jurgen Wischnewski, sent personally by West Germany's former chancellor, Willy Brandt, toured the region. After stopping in Mexico for talks with the FDR, his mission nearly aborted in El Salvador when the junta showed him a captured guerrilla document, dated Feb. 3, titled "The Negotiation Maneuver." Feeling betrayed, Wischnewski refused to turn around to talk to the FDR again and proceeded to Costa Rica and Panama.

At the suggestion of Panamanian strongmen Gen. Omar Torrijos, however, Wischnewski, according to a source close to the envoy, then visited Cuba. There, President Fidel Castro reportedly reassured him that the guerrilla document was only an "option paper," which was not adopted as policy. g

Castro, whom the United States has charged with supplying weapons and logistical assistance to the Salvadoran guerrillas, assured Wischnewski that the FDR was serious about negotiating, and that he would support any mediation effort and whatever settlement the Salvadoran left would accept, the source said.

Castro also confirmed, the source said, that Cuba had indeed sent arms to El Salvador, but had stopped, adding that the operation had been entirely his decision and had not involved the Soviet Union.

At the end of the Wischnewski tour, the warning sides were to have deposited their positions papers on talks in the Mexican Embassy in San Salvador. But the mission came to an abrupt end when the Salvadoran junta, without further contact with Wischnewski, publicly rejected the notion of outside mediation.

While the pro-FDR Socialist International had less chance to deal neutrally with both sides in the Salvadoran conflict, a plan for Mexico, whose governing party has openly sided with the FDR, and Venezuela, which has been the strongest junta backer outside the United States, to promote talks appeared more viable.

Both countries, so it seemed, could influence one of the warring sides in a way few others could. Mexican President Jose Lopez Portillo, whose country is most vulnerable to prolonged instability in Central America, reportedly was keen to present a joint Mexican-Venezuelan position on how to deal with the civil war during his meeting with President Reagan, originally scheduled for late April and now planned for this weekend in Washington.

After Venezuelan President Luis Herrera Campins visited Mexico in April, he and Lopez Portillo once again called for a political solution in El Salvador, offering their good offices. But privately, according to a Mexican official, Herrera Campins told Lopez Portillo "he did not want a mediation effort now because it implied strengthening the guerrillas and could erode Duarte's position as president."

Venezuela has since proposed broadening the Salvadoran junta prior to the elections, the source said. That idea has already been turned down by El Salvador.