The Salvadoran leftist leader who met with U.S. special envoy Richard Stone said tonight that things are "moving in the right direction" for early talks with an open agenda between Stone and El Salvador's rebel leadership.

Ruben Zamora, who saw Stone yesterday in Bogota, Colombia, portrayed their first contact as a promising start opening the way for "dialogue" long sought by El Salvador's leftist movement but rejected by the U.S.-backed government as capitulation to guerrilla violence. He said further talks, which he strongly suggested were imminent, will have "an open agenda."

Reagan administration policy has been that any dialogue would have to be confined to discussing ways for the rebels to participate in Salvadoran elections to be organized by the U.S.-backed government. There was no comment from U.S. officials in Central America, and Stone went on to Nicaragua today for talks with the Sandinista leadership.

Stone held hour-long talks with Nicaraguan leaders and described the encounter as "very useful" He then returned to Washington, ending his 10-day Central American tour, The Associated Press reported.

In Washington, State Department spokesman John Hughes said "there may be other meetings" between Stone and Salvadoran opposition leaders but did not say when or where they might take place, United Press International reported.

"It is only a small step, a first step," Zamora said in a conversation with reporters at San Jose's international airport. "But it is a step in the right direction."

Zamora is a leader in the political arm of the five-group organization known as the Farabundo Marti National Liberation Front. His two-hour meeting with Stone marked the first formal contact between President Reagan's special Central American envoy and the guerrilla leadership fighting to overthrow the U.S.-backed government in San Salvador.

President Belisario Betancur of Colombia, who helped arrange the contact, hailed it as "a new effort in the process of seeking peace in Central America," addressing journalists in Bogota with Stone and Zamora silent at his side.

Zamora cautioned tonight, however, that the Bogota meeting was only "preliminary." He said it was designed to remove obstacles to a broader meeting between Stone and the full rebel leadership, presumably including the top official, Guillermo Ungo. A previous attempt to set up such a meeting here in the Costa Rican capital last month fell through over what the guerrilla leadership said were procedural disputes.

Citing an agreement with the United States that the contacts would remain secret, Zamora declined to reveal the contents of yesterday's successful talks. But in response to questions, he declared that broader talks were agreed upon and expressed optimism that the disagreements that led to last month's failure have been overcome.

"The decision for a meeting for an open agenda has already been arranged," he asserted.

The guerrilla leadership repeatedly has sought to open negotiations with the United States and the Salvadoran government on a political solution to the 3 1/2-year-old civil war. But the Salvadoran government, armed and supported by the United States, has maintained that there can be no talks on "power sharing" with the guerrilla movement, insisting that only through elections can the left gain a share of power in El Salvador.

It was unclear whether Zamora's assertion that the upcoming talks with Stone would have an "open agenda" meant the United States has softened that position, or whether both sides simply agreed to approach the problem of what to talk about after the talks begin. Zamora hinted at the latter, saying that while negotiations are a "strategic" goal for the rebels, they are only "tactical" for the Reagan administration as it seeks to meet congressional demands for flexibility.

"On the one hand, Ambassador Stone and the American administration are talking of dialogue," he said. "But on the other hand, they are taking all the practical steps for a military solution."

Zamora's comments also pointed out another major divergence between the way the Salvadoran rebels view the upcoming "dialogue" and the way the Reagan administration views it.

Stone, a former Florida senator, is in Central America as a representative of the U.S. president and not to carry messages or arrange contacts between the rebel movement and President Alvaro Magana's government in San Salvador, he insisted. "We do not consider Mr. Stone to be a mediator between the Salvadoran government and us," he said. "Let us make that quite clear."

U.S. officials have depicted Stone's mission as an effort to draw the guerrillas into talks with Magana's government aimed at leftist participation in elections.

In that view, the talks would center on such subjects as guarantees for the safety of leftist leaders running for office and would avoid consideration of guerrilla goals of a coalition government with members of the rebel leadership.

But the guerrilla movement has contended that the United States, by its extensive support for the San Salvador government, has become a major party to the civil war that must be negotiated with directly to arrange a solution.

In Tegucigalpa, meanwhile, rebels fighting Nicaragua's Sandinista government called Monday for their participation in regional peace talks, saying they are "pacifists" who have been overlooked by the diplomats, AP reported. "We feel there is a double standard," said Adolfo Calero Portocarrero, one of seven directors of the Nicaraguan Democratic Force. Its Honduras-based guerrillas have been mounting raids into northern Nicaragua since the first of the year.