A spokesman here for the guerrilla forces in El Salvador yesterday called for direct U.S. participation in negotiations among the warring parties there, saying presidential envoy Richard B. Stone would be acceptable as a participant but not as a mediator.

Ruben Zamora, North American spokesman for the joint command of the Farabundo Marti National Liberation Front and the Democratic Revolutionary Front (FMLN/FDR), reiterated at a meeting of the Disarm Educational Fund, a New York-based educational organization, that his group would not take part in any elections under existing political conditions in the Central American nation. Instead, he listed elections as one of six points on a proposed agenda for talks aimed at "a comprehensive political settlement." The Salvadoran government and the Reagan administration have repeatedly rejected the idea of discussions over power sharing.

Zamora's visit to Washington was hurriedly arranged, apparently to coincide with the arrival here yesterday of interim Salvadoran President Alvaro Magana, who is expected to announce dates in November and December for two-stage presidential elections when he meets today with President Reagan.

Zamora called as he has in the past for direct dialogue among all "parties directly involved" in the 3-year-old conflict in El Salvador, but for the first time he included the United States in that group.

"Because of its role in providing economic, political and military support to the Salvadoran regime, as well as its increasing control over the decisions of the Salvadoran government, we consider the Reagan administration as a belligerent party directly confronting" the FMLN/FDR, said a prepared statement Zamora distributed.

"While our conflict has its roots in the injustice and repression suffered by the people, the increasingly militaristic and interventionist role of President Reagan's administration demonstrates that in El Salvador there will be no peace, no justice and no independence as long as this policy continues," the statement said. Stone, Zamora said, is "a representative of one of the parties directly involved in the conflict and therefore not a mediator." Stone said during Senate hearings on his nomination that he would be willing to meet with the FMLN/FDR to help mediate a settlement to the Salvadoran conflict.

Zamora listed six points that he said the FMLN/FDR wanted on the agenda, although he said the guerrillas have no order of preference or of discussion. He indicated willingness to accept other issues. The topics would be socio-economic reforms, the future of the government and guerrilla armies, the composition of the provisional government, future elections, the international policy of El Salvador and a cease-fire.