The Salvadoran guerrillas are ready to negotiate an end to the Salvadoran war through talks with all sectors of that country's leadership, including the U.S.-backed junta, the Army and businessmen, according to a senior insurgent commander speaking for the guerrilla leadership.

The proposal for negotiations without prior condition is more detailed and wide-ranging than earlier initiatives. It comes at a time when the alternative to talks appears ever more likely to be prolonged war that endangers the stability of the entire region.

In a clandestine interview late last month in another Central American country, a woman identified only as the number two commander of the Popular Liberation Front and member of the joint directorate that coordinates the five guerrilla factions said the insurgents want "a dialogue together with all interested sectors -- all of them -- including the junta, the military, private enterprise, everyone."

The guerrilla commander would not reveal her name nor was it available from other sources. The interview was arranged by civilian leaders of the Salvadoran opposition to clarify the position of the insurgents' high command, which is often considered more radical than its noncombatant allies and spokesmen.

The interview's basic points were subsequently confirmed by Fabio Castillo, a civilian member of the leftist opposition's political-diplomatic front.

U.S. diplomats with whom the plan was discussed said privately that it was an interesting change in the left's position but they were not in a position to comment on it in any official capacity.

Both the Salvadoran government of Christian Democratic President Jose Napoleon Duarte and its backers in the Reagan administration repeatedly have rejected any negotiations affecting the makeup of the current leadership. They insist that elections scheduled for March are the only political solution.

A proposal for negotiations put before the United Nations in early October by Nicaragua on behalf of the guerrillas called only for talks with the junta. The proposal to negotiate with all sectors is an elaboration of that plan. Both call for unspecified governments to witness the talks. The earlier one, however, set an apparent precondition, requiring accord by the two sides on an agenda.

Several Latin American and European countries have expressed interest in aiding a negotiated settlement. But an initiative by France and Mexico, like that of Nicaragua, has met with little success because of opposition to negotiation by the United States and the junta it supports.

Salvadoran and U.S. opponents of negotiations argue that the guerrillas are merely attempting to buy time or to win at the negotiating table what they cannot gain on the battlefield or by the ballot.

The insurgent leader would not directly confirm reports circulating in diplomatic circles and the Salvadoran countryside that the guerrillas are planning a substantial increase in their level of activity early next year. She said, however, that if the current proposal for negotiations is dismissed, "the war will deepen."

The conflict already has spilled across the Honduran border and is the root of increasingly belligerent confrontation between the United States and Nicaragua, which the Reagan administration accuses of acting with Cuba to supply the Salvadoran rebels.

"Those who say we are asking for a political solution because we are weak and afraid to continue the military line, I think they are making a mistake," said the commander. "We do not want to put an end to the war just to make an end. That's clear."

Since the largely failed offensive by the guerrillas last January, they have adopted a Vietnam-style war of attrition, improving the "quality" of their operation, as the woman put it. She said they are now ready to increase the "quantity." She and independent sources who recently visited guerrilla camps said that the insurgents now have, in effect, a uniformed and well-supplied army.

The outlines of the government that the guerrillas hope to achieve through negotiation have remained largely unchanged and could indeed put the country under their control. But they are general enough to allow considerable discussion, and none of the desired changes was stated as a precondition for the start of talks.

The vaguely stated goals include some that would not appear objectionable to the other side but others that could prove difficult for the junta to accept. According to the woman, the guerrillas would look for a reintegrated government including representatives of the insurgents, their political allies and others "interested in resolving El Salvador's basic problems."

National independence and self-determination would be guaranteed, political, social and economic changes would be implemented to guarantee more democratic participation. The Army would be restructured to include former guerrillas and government soldiers not directly implicated in what she called "the genocide."

Freedom of religion would be guaranteed, under the future government, she said, and private enterprise would continue to exist in the context of a mixed economy. Foreign policy would be nonaligned.

Elections would be held, the guerrilla leader said, but the purpose of the negotiations should not simply be to have elections. Without elaborating, she dismissed the vote scheduled by the junta for March as another "tool of repression."

The guerrilla spokeswoman seemed to recognize the unacceptability of some such positions for the Salvadoran military, and she cited this as one reason for offering to meet with all affected sectors and for placing no prior conditions on talks.

"We don't want one-sided negotiations that don't express the interests of the different sectors," said the guerrilla spokeswoman.

But in recent months the guerrilla forces have appeared to be steadily gaining strength. To combat even the present force and tactics of the insurgents, the Salvadoran government needs ever-increasing U.S. economic and military aid.