Private refugee-aid organizations have filed suit here against the federal government to stop what they say are illegal efforts to force refugees from war-torn El Salvador to return home.

The suit was accompanied by formal complaints to the Organization of American States and the United Nations as part of a national legal and diplomatic campaign to protect thousands of Salvadoran refugees arriving here illegally each year.

Peter Schey, an attorney with the National Center for Immigrants' Rights Inc., charged the U.S. Immigration and Naturalization Service with "physical abuse, intimidation, threats and coercion" in dealing with the approximately 400 Salvadorans forced back out of the United States each month.

The suit, filed as a class action in U.S. District Court, said immigration officials coerced refugees into accepting voluntary departure immediately after their arrests by threatening them with long jail stays or an end to any chance of immigrating legally if they refused.

It also said the service violated its own rules by ignoring refugee requests for political asylum and hindering refugee contacts with lawyers.

Omer G. Sewell, deputy district director for the immigration service here, called the charges "absolutely false."

He said agents explained to illegal immigrants that refusal to sign a voluntary-departure form would mean spending additional time in custody, but those who refused to sign were allowed access to lawyers and could remain in the United States for deportation hearings and requests for political asylum.

The suit argues that Salvadorans should be eligible for political asylum because an estimated 12,000 persons were killed in politically inspired violence in El Salvador in 1980 alone.

It notes that the U.N. high commissioner for refugees has suggested that all El Salvadoran refugees be regarded as qualified for political asylum unless clear evidence to the contrary exists.

Schey said an estimated 8,000 have requested asylum here and about 2,000 of those have been turned down, with only two requests granted last year.

Schey charged that federal officials have decided that honoring such asylum requests would be at odds with the stated position of the Reagan administration that the government in El Salvador is not violating human rights and is thus entitled to U.S. aid in fighting a leftist insurgency.

Paula Kuzmich, spokesman for the State Department's Human Rights Bureau, said the department did not keep statistics on the number of requests it approved.

She acknowledged, however, that State Department officials have said that most Salvadoran refugees, who usually cross the border illegally from Mexico, are "economic refugees" seeking jobs and not escaping political persecution.