Southside United Presbyterian church sits in a scruffy, heat-bleached neighborhood next to part of the city dump. Inside, a 10-foot cross hewn from worn out railroad ties looms over a simple meeting room equipped with only an organ and wooden chairs.

Here, on March 24, 1982, the Rev. John M. Fife and his congregation invoked a centuries-old Christian tradition and declared their church a "sanctuary" for illegal Salvadoran and Guatemalan immigrants.

It was a solitary, symbolic gesture in defiance of federal laws that forbid helping undocumented aliens flee immigration authorities.

Two years later, more than 150 parishes and congregations from Massachusetts to California -- including seven in the Washington area -- have joined Southside in what has become an increasingly popular form of grass-roots protest of U.S. policies in Central America and the federal government's treatment of civilians caught up in the conflicts there.

Organizers say that 50,000 people have participated in the sanctuary movement in one way or another, and that several hundred Central Americans have sought refuge through its network of churches. This figure represents only a fraction of the estimated 500,000 illegal aliens in the United States, but total numbers are not the major concern of a movement whose power is meant to lie mostly in its symbolism.

What makes the sanctuary movement different from past protests is that it is challenging Reagan administration policies in Central America through an unusual forum: U.S. immigration law.

The movement's close ties with the church pose a dilemma for the U.S. Immigration and Naturalization Service, which by law is required to apprehend illegal aliens. Wishing to avoid a public clash with churches and hoping to minimize media attention to the movement, INS has studiously ignored its activities for the past two years, including a widely publicized caravan of cars that transported an undocumented Guatemalan family from Chicago to Weston, Vt. last April.

"I don't think anybody, Congress, the White House or the public, wants us going into churches looking for illegal aliens, and it's just not practical or worthwhile for us," says INS spokesman Verne Jervis.

Members of the sanctuary movement contend that the administration, for political reasons, has deliberately misapplied immigration law in deciding whether Salvadorans should be allowed into this country.

"The movement," says Fife, "has created an awareness nationally and in Congress and the administration that Central American refugee issues are an important national consideration . . . and established a clear linkage between immigration and refugee issues and the policies of the United States in Central America."

Typically, the Central Americans, using assumed names and handkerchiefs or sunglasses to conceal their identities, have been presented to reporters during a dramatic welcoming ceremony at which they recite tales of persecution and mistreatment at the hands of army or police in their own countries.

This is a prelude to an extended period during which the churches provide food, clothing, employment and housing, sometimes, but not always, in church-owned buildings.

The major impetus for the movement has come from religious communities across the country and made up of people of all ages and income groups.

So far it has been endorsed by the American Friends Service Committee, the Conservative Rabbinical Assembly, the General Assembly of the United Presbyterian Church U.S.A, the United Methodist Board of Church and Society and the Board of National Ministries of the American Baptist Church U.S.A.

The motives of many sanctuary supporters lie at the intersection of religion and politics. "The suffering of the people of Central America is so great it demands a response from people of faith here," says Benedictine Brother Philip Fronckiewicz of Weston Priory in Weston, Vt., which is providing sanctuary to a Guatemalan family of seven.

"It raises religious and moral questions for us and it was a moral and ethical response for us to become a sanctuary," said Fronckiewicz.

Others put politics first. "One of our goals is to end U.S. intervention in Central America," says Lee Holstein of the Chicago Religious Task Force, an ecumenical group that acts as a clearinghouse. The movement "provides safe and public forums for refugees from El Salvador, Guatemala and Honduras so they can speak directly to the American people in the United States about conditions in those countries, about why they left and what the U.S. is doing there."

Those involved in the movement define their role in different ways. "To some people the sanctuary movement is helping undocumented aliens evade the authorities; to some it is transporting them; to some it's helping them with food and shelter," says Roman Catholic Bishop John J. Fitzpatrick of Brownsville, Tex.

In recent months, three activists in the movement have been charged with transporting undocumented aliens. INS officials say all three were apprehended as the result of routine border patrol operations and were not targeted specifically because of their involvment in the sanctuary movement.

Despite its roots in Christian tradition, the status of sanctuary as a legal concept is unclear in modern U.S. law. The INS says that members of the clergy are not immune from federal immigration laws and that with a proper search warrant they can enter a church to apprehend illegal aliens.

"Some things done by the so-called sanctuary movement are legal and some are not," says Hal W. Boldin, INS district director in Harlingen, Tex. "It's perfectly legal to give food and shelter. Harboring an undocumented alien is only a violation if they are being concealed . . . trying to catch someone giving food and water to illegal aliens, it's just not our operation."

But "transporting of an alien who entered the U.S. illegally in furtherance of that illegal entry is a violation," he says. "We're not talking about sanctuary there."

The political implications of granting sanctuary are only too apparent to the INS. "The only debate going on is a political one and it has to do with an attempt to change the foreign policy of the U.S. in El Salvador," says Boldin, who calls the illegal immigrants "pawns" of the critics.

At issue is whether the Salvadorans who come here are mostly economic migrants as the federal government claims, or political refugees, as many of its critics claim. Classified as refugees, Salvadorans would be allowed to stay here, at least temporarily, even if they had entered the country illegally.

In the past 22 months the government has granted political asylum to only 391 Salvadorans and denied it to 13,790, saying they failed to prove that they personally were persecuted in El Salvador.

But many religious groups, refugee groups and immigration lawyers, armed with numerous case histories to back them up, charge that INS and the State Department have turned down many Salvadoran applicants with valid stories of personal persecution because the United States did not want to cast the Salvadoran government in a negative light.

"To some degree," says Roger Winter, director of the U.S. Committee for Refugees, a private group, "the U.S. has precipitated the problem by its own unwillingness to keep politics out of the [asylum] system. If back in those early years the U.S. asylum system had adequately distinguished between people who had reasonable cases and people that didn't, then a lot of these later complications, including the sanctuary movement, might not have grown up quite like it did."

Organizers of the movement operate on the assumption that all Salvadorans and Guatemalans are refugees, though they admit they cannot know for sure if the stories they tell are true. "We try to do an extensive screening process and get corroborating letters from churches or the United Nations High Commission for Refugees," says Philip Conger, a refugee program worker at Southside.

"It's difficult for someone to maintain a story that's coherent over a period of time if they are not telling the truth. We have rejected some people we thought were not telling the truth," Conger says.

The first stop for most Salvadorans crossing into Arizona is Southside Church. But before that many of them have met Jim Corbett and his wife, Patricia. A retired rancher and a Quaker who holds a master's degree in philosophy from Harvard, Corbett, 50, says a chance encounter with a Salvadoran refugee three years ago got him interested in Central America.

Since then the Corbetts have been involved in what they call "evasion services," helping Salvadorans and Guatemalans evade the U.S. Border Patrol as they cross into the United States from Mexico. They have helped about 1,000 to do so since 1981, Corbett says. Not all of those go into the sanctuary movement.

Corbett travels to Mexico about every three months to contact an informal network of priests and human rights groups helping Salvadorans and Guatemalans living there. Seeking out people he considers most needy of refugee protection, he makes arrangements for them to cross the border, advising them where to do it and where to meet him after they enter the United States.

Generally the Central Americans are matched with churches by the Chicago Task Force, which runs a nationwide network of contacts who transport them from one place to another. Margaret Volpe of Davenport, Ill., is one of those contacts.

"There's a woman in Nebraska who does routing -- she calls us and we send someone to pick them up at such and such an agreed point," says Volpe, a 39-year-old Catholic. "We have taken them to next point, which is usually Chicago. Usually we meet at a rest area, on a highway or sometimes at a home."

Darlene Nicgorski, an American Franciscan nun who worked in Guatemala for 10 months, is another of those contacts. Working out of her apartment in Phoenix, she screens potential candidates for the sanctuary movement.

Nicgorski says she must determine if they have the stamina and ability to cope with the publicity and with the strains of living in a community where they may be the only Hispanics. Most who enter the movement do so in the hope that by speaking out, "they are helping people who can't get out," Nicgorski says.

This is the reason given by Pedro, a 29-year-old illegal alien who is now staying with Nicgorski in Phoenix. In a telephone interview, Pedro says through an interpreter that he was a photographer for the Salvadoran Human Rights Commission and helped retrieve the body of its president, Marianela Garcia Villas, after she was slain in El Salvador in March 1983. After soldiers came to his home looking for him, Pedro says, he feared for his life and fled to Mexico City and worked with the commission's offices there.

Although the Mexican government "has much respect for our work," immigration officials and intelligence agencies "made people like us feel uncomfortable," he says. So he came to this country a month ago, crossing the border clandestinely. He plans to enter the sanctuary movement "to explain the way the assistance being sent to El Salvador is being used . . . and in this way the American public will know their president is helping a government that is killing the people."