SAN SALVADOR, El Salvador (AP) _ Hundreds of shouting supporters greeted leftist civilian leader Ruben Zamora, who returned temporarily to El Salvador Saturday after seven years of exile ''to be in the people's fight.''

Zamora, vice president of the Democratic Revolutionary Front, took the blue-and-white Salvadoran flag in his hands and said with obvious emotion, ''I clutch the flag of the fatherland, this is the only amnesty that I take shelter in.''

That was an apparent reference to an amnesty program enacted by the government of President Jose Napoleon Duarte to comply with part of a new Central American peace plan. Hundreds of political prisoners have been released in the last two weeks under the program.

In the past, Zamora has visited El Salvador under ''truce protection'' to take part in peace talks with government representatives. But this is the first time he entered the country as an ordinary citizen and political leader.

Zamora kissed the flag while supporters shouted, ''The people united will never be defeated.''

The Democratic Revolutionary Front is the political ally of the Farabundo Marti National Liberation Front, a coalition of five guerrilla groups fighting the pro-American government of El Salvador since October 1979.

Front president Guillermo Ungo, also living in exile, was scheduled to return to El Salvador on Monday. At a news conference, Zamora said the length of his stay will depend ''on the circumstances that develop during my visit.''

Zamora also said he was fearful about returning to his country because of recent slayings, including the Oct. 26 killing of Herbert Ernesto Anaya, president of the Human Rights Commission.

He was accompanied by six Americans, members of the Unitarian Universalist Service Committee, and three Central American politicians.

Shortly after his arrival, Zamora met with Archbishop Arturo Rivera Damas, leader of the Roman Catholic Church in El Salvador and mediator of peace talks between the government and the guerrillas.

Zamora has said he and Ungo planned to meet with representatives of political parties, trade unions, professional and business groups and university organizations. They had no plans to meet with government officials.

Zamora, who arrived on a Nicaraguan Aeronica flight from Mexico City, was met by about 500 members of labor, university and professional groups. They carried red-and-yellow banners of the Socialist Christian Popular Movement, the party Zamora headed before he fled the country in early 1980.

''I come after seven years of exile in order to be in the people's fight,'' said Zamora, who has been living in the Nicaraguan capital of Managua. ''No one is defeated here. What there are here are fighters.''

He called on his backers to unite because ''the future of the country is what is at stake.''

Supporters of the government and the guerrillas in this country of 5.2 million distrust each other after eight years of kidnappings, civilian slayings and a U.S.-financed air war. Church and human rights groups put the death toll at more than 63,000, most of them civilians.

Zamora told reporters aboard his flight that the guerrilla units may declare a five-day cease-fire to show their approval of his stay in the country.

A former member of a civilian-military junta formed in 1979, Zamora fled El Salvador in 1980 after his brother, Mario, was assassinated by a rightist death squad.

Ungo resigned from the junta soon after it was formed and fled the country within months under pressure from rightist groups within the military. He has been living in Panama.

The way for their return was paved by the Central American peace plan, signed Aug. 7 by presidents of El Salvador, Nicaragua, Costa Rica, Honduras and Guatemala. It calls for cease-fires, amnesties, reconciliation between governments and opposition groups, and reforms to bring about greater democracy in the area.

The rebels and government representatives met twice in October to arrange a cease-fire as part of the peace plan. Rebels refused to attend a third set of talks to protest the slaying of Anaya. Two other rounds of talks between the government and the rebels in 1984 ended in deadlock.