PANAMA CITY, JULY 14 -- The government, in a sudden shift, decreed an amnesty today for Panamanians arrested during protests last Friday. Judicial authorities earlier had told dozens of relatives that the prisoners had been sentenced to as long as 18 months in jail.

This evening, more than 100 surprised detainees were released from the Modelo prison. It remained unclear whether others in the group were still held.

The amnesty calmed the growing alarm among family members of Panamanians who had remained behind bars for four days. There were many reports of beatings by Defense Forces troops, and some detainees released today made new allegations of mistreatment. During five weeks of turmoil, thousands have been arrested. But until this incident they had been released after a few hours.

The longer detentions and threats of stiff sentences appeared to be the Defense Forces' tactics to intimidate protesters calling for the military commander, Gen. Manuel Antonio Noriega, to step down.

The measures seemed to have an effect. A call by the opposition National Civic Crusade, a coalition of business and professional groups, for a handkerchief-waving demonstration on the city's sidewalks this afternoon drew a small turnout.

About 50 family members, most of them black, had gathered at the Chamber of Commerce this morning to form a committee to press for the prisoners' release. They said many of their relatives were not involved in the opposition march Friday but were caught on the street in police roundups.

Ignacio Garcia, 37, a mill chief at a General Mills flour plant here, was picked up as he emerged in his white factory uniform from a bar, his sister Graciela said. White handkerchiefs are the Crusade's emblems.

A judge told Graciela Garcia early today her brother was sentenced to a year in jail for sedition and vandalism. He was freed at 6 p.m.

The Chamber of Commerce has been at the center of the anti-Noriega protests. Until now, most Panamanians who came to its meetings were white. In a television interview Sunday, Noriega called the opposition racist and dismissed the protests as the work of 5,000 members of the light-skinned upper class.

Riot police prevented the Crusade from holding a major rally Friday. The mainly middle-class opposition is calling for Noriega to step down because he has been accused of election fraud and assassination.

Between Sunday and noon today, the Chamber of Commerce received 221 complaints from relatives of detainees, said its vice president, Raul Mendez.

The amnesty decree had the markings of a quick decision by Noriega, who in practice controls the government. Spokesmen for President Eric Arturo Delvalle had no information about the decree as it was being announced on television as having been signed by the president.

Before dawn, dozens of relatives were summoned to a night court and read lists of prisoners' names and sentences ranging from six to 18 months. There was no trial or hearing, the relatives said, and no lawyers were present at the court or the prison.

Student Jose Benjamin de Gracia, 19, said in an interview after his release that police in a passing van jumped out and seized him Friday. "I was crossing the street. That was my crime," said the bruised youth, sobbing. He said he was taken to three different Defense Forces garrisons Friday afternoon and was kicked and punched.

One uniformed soldier who discovered de Gracia had twisted an ankle during his arrest kicked him in the ankle, he said. De Gracia also charged that prison guards forced him to watch as they stripped another prisoner, put their boots on the man's neck and beat him with rubber hoses. Police found a white handkerchief in de Gracia's pocket and told him it constituted evidence against him, he said.

Leida Ortaz, 31, who is six months pregnant, said that when her husband, factory worker Carlos Castillo, went out Friday to buy her some medicine, police fired on him and beat him with rubber truncheons.

Ortaz charged that when she came out to help her husband she was beaten on the legs and back. She exhibited bruises today. She said she took her bleeding husband to a public hospital emergency room and went herself to the maternity ward for treatment for stomach pains. When she returned, she said, police had removed her husband from the emergency room and taken him to prison. She could not be reached tonight to confirm his release.