El Salvador's experiment with amnesty has given freedom to nearly 500 political prisoners but enticed only a handful of guerrillas into turning themselves in to the U.S.-supported government.

At the halfway point of its 60-day term, the amnesty thus illustrates the difficulty of any attempt to draw antigovernment forces into peaceful politics under the present government, as the Reagan administration seeks to do. At the same time, it dramatizes the extent of fear and mistrust that many Salvadorans feel for their institutions after three years of often savage civil war.

"Any amnesty law, if it is really serious, can play a beneficial role," said Ernesto Arbizu Mata, a lawyer and former government official who heads the three-man Amnesty Commission. "But in a country like ours, it is difficult. You can receive surprises from all sides."

Since the constituent assembly unanimously approved the law on May 4, 496 Salvadorans have been released from prisons where they were being held on political charges, out of a total of 700 when the amnesty began. Although El Salvador has had three previous amnesties since 1979, the number of those freed this time marks a departure in the treatment of political detainees.

The amnesty law was drafted as part of the government's effort, at the urging of the Reagan administration, to hold elections and bring opposition figures into the political process.

Under the law, any political prisoner who has not been tried or is serving a sentence of less than four years may apply to have his case reviewed by a military judge. If amnesty is granted, the commission is to assist the former prisoner in leaving the country or returning safely to civilian life. Guerrillas who turn in their arms also may apply for amnesty.

Many of the 200 political prisoners remaining in custody were not eligible for amnesty or were considered such important opposition figures that they could not be released. Many other cases are still under consideration by judges.

According to the commission's count, only 56 of those released have come to the commission offices seeking the assistance that the commission was set up to provide. Of those, about 40 have requested help in getting a passport to leave the country rather than in getting a job to settle back into society.

Many others have arranged to flee El Salvador through foreign embassies. Arbizu said Spanish, Belgian and Mexican diplomats here have arranged for a number of discreet departures, including that of Commander Clelia of the Farabundo Marti National Liberation Front, who was released two weeks ago.

Immigration officials have reported issuing about 200 passports to freed prisoners on their way out. Others are believed to have fled on passports they already carried. According to the law, the commission was to provide special cards to surrendered guerrillas or prisoners released on decision of a judge. The cards were to serve as identity and official proof that the bearers were no longer at odds with the law, allowing them to remain safely in El Salvador.

"For us, this is not a guarantee," said Maritza Ruiz of the Committee of Mothers and Family Members of Political Prisoners, Disappeared People and Assassination Victims.

"We don't have confidence any more. The mistrust we have is that coming out of prison, they can be captured again or killed. This is what we are afraid of."

Ruiz said she has a brother and sister who were released under the amnesty law. Neither has gone to the amnesty commission for help, she added in an interview, because they fear what might happen to them if they go out in the street.

According to the relatives' committee, four men released under the amnesty law last month were found later with their heads cut off and another was picked up by armed men last Monday, the day after he left prison. Arbizu said he knew of no such cases.

Government critics charge that the law means little in the long run because authorities are arresting other prisoners almost as fast as they are releasing prisoners who have received amnesty. Fourteen persons were arrested recently in the western town of Ahuachapan, for example, on the basis of information linking them to the outlawed People's Revolutionary Army, one of five component groups of the Farabundo Marti National Liberation Front fighting to overthrow the government.

Col. Reynaldo Lopez Nuila, head of the National Police, said in an interview that about 50 persons have been arrested on political charges since the amnesty began.

Arbizu said only 61 guerrillas have turned themselves in for amnesty.

Several dozen former rebels are being protected at the international fairgrounds in San Salvador, where the commission has its offices. In one indication of what they face if they remain in El Salvador, Arbizu asked reporters interviewing them to conceal their names.

One 25-year-old farmer said he spent six months with the People's Revolutionary Army in the far eastern province of Morazan.

Like most of those interviewed, the former guerrilla said he is hoping for a visa to go to another country, fearing for his life if he remains in El Salvador. Canadian diplomats last week began interviewing candidates for political asylum, but the United States has yet to become involved, Arbizu said.

One candidate at the fairgrounds, a youth named Oscar, said he was a political organizer for the People's Revolutionary Army who narrowly escaped arrest in the recent Ahuachapan roundup. His goal now, he said, is to get out of the country and then return clandestinely to resume his revolutionary work.