MANAGUA, NICARAGUA, SEPT. 23 -- Salvadoran rebel leader Ruben Zamora, after more than seven years in exile, is packing his bags for two trips to his country: one for peace talks with the government in early October and a second to return home for good.

Today, the leftist rebel alliance formally agreed to a third round of talks with the Salvadoran government Oct. 4, in a whirlwind of peace-making activity in Central America. The activity coincided with the visit of Costa Rican President Oscar Arias, author of a regional peace plan, to Washington and the United Nations, where he spoke today.

The Salvadoran peace talks, the first since November 1984 between the government and the leftist rebels, will take place at the Vatican mission in San Salvador, following an invitation made yesterday by President Jose Napoleon Duarte.

Zamora, a leader of the rebels' political wing, said in an interview today at his Managua home that the rebel alliance sent a letter two weeks ago to Arias praising the Central American peace plan's general goals of reconciliation through dialogue.

The peace plan was signed Aug. 7 in Guatemala by the five Central American presidents.

The letter, and a series of phone calls from Arias, were enough for Duarte. Earlier the Salvadoran president had balked at holding talks with the rebels when they refused to accept the specifics of the Guatemala plan, which calls on insurgent armies to renounce warfare and accept an amnesty.

It remains to be decided whether Duarte himself will attend the meeting and what topics will be raised. El Salvador's Roman Catholic leader, Archbishop Arturo Rivera y Damas, is to mediate.

Despite the plans for talks, the two sides appear to be on a collision course.

The rebel front -- made up of five guerrilla forces and a coalition of civilian groups -- holds to its three-year-old demands for a negotiated transition government, incorporating some guerrilla leaders, to run the country until a new round of elections can be held, Zamora said. The Duarte government has insisted that the guerrillas join the existing political process, waiting for the next scheduled elections in 1989 to run for office.

"In El Salvador there are two powers," Zamora asserted, referring to the government and the guerrillas. "Duarte can't just ignore that." He said the rebel coalition will ask the government to guarantee the safety of its civilian members who wish to return to their homeland.

Zamora said his plans to move his household and political fortunes back to his country are independent of the talks. His party, a breakaway from Duarte's Christian Democrats, decided in 1985 to start sending its members one by one back to San Salvador to test the waters. In earlier years, returning opposition figures have been assassinated by death squads, which allegedly have been connected to the armed forces.

Of dozens who returned more recently, no one has been harmed by political killers nor harassed by the armed forces, Zamora said. Now they want him, their secretary general, to take the same risks. Zamora fled El Salvador in March 1980, after his brother Mario was killed by a death squad.

Zamora has not set a date for his return, but it will be before Nov. 7, the deadline for regional compliance with the Guatemala accord, he said.

Although he said he will have to live constantly on guard against attackers, Zamora acknowledged that El Salvador seems safer for opposition activists than it was seven years ago.

"I'm going back to be a politician, not a martyr," he said.

He will be walking a political tightrope, exploring the opportunities for civilian political activity while retaining his alliance with the guerrillas.

"It's a political adventure, a classic case of something you can't know until you try," he mused.