A radio message from 29 American peace activists, reportedly detained yesterday by anti-Sandinista guerrillas while traveling in a boat along the Nicaraguan-Costa Rican border, indicated they had been freed and were in good health, a spokesman for the group said today.

Sharon Hostetler, a leader of the U.S.-based ecumenical organization Witness for Peace, said at a press conference that the radio message at noon today said the Americans were once again navigating up the border formed by the San Juan River after having been detained for 29 hours by armed men who identified themselves only as Nicaraguans belonging to "an independent anticommunist group."

Witness for Peace has sponsored trips to Nicaragua for about 1,500 Americans on the theory that the presence of Americans might prevent rebel attacks.

Meanwhile, Acting Foreign Minister Victor Hugo Tinoco reiterated a Sandinista proposal first made last year that would establish a demilitarized zone along the 200-mile border and would effectively force all rebels out of that area.

[In Washington, the State Department said the Costa Rican Civil Guard had reported that the group stopped in the Costa Rican border town of Las Tiricias and told the local commander that they were all well. They said they had decided to go to the Nicaraguan river town of El Castillo de la Concepcion and spend the night there, the State Department said. A spokeswoman said the group arrived in the town tonight, the Associated Press reported.]

The region of the San Juan River has been the scene of heavy fighting, especially during the past three months, between the Sandinistas and members of the Revolutionary Democratic Alliance, or Arde, loyal to former Sandinista commander Eden Pastora.

The Sandinistas announced last week that air strikes and ground attacks that began in May had wiped out all of Pastora's camps along the river in Nicaraguan territory and chased many of his forces across the border into Costa Rica.

The 29 U.S. citizens -- accompanied by an undetermined number of journalists, some of them also Americans -- began their trip Tuesday in a demonstration they labeled "Flotilla for Peace."

The group navigated downstream from the town of San Carlos, 150 miles southeast of Managua. They camped near the river bank that night and were on their way back upstream at 7:45 a.m. yesterday when they reported a shot over their bow, according to a transcript of a radio communication read by Hostetler. According to the transcript, the Americans were then ordered by three armed men to disembark on the Costa Rican side of the river.

In another communication last night, radio operator Warren Armstrong, 25, of Wayne, Pa., said everyone was fine.

"We are waiting for orders from our brothers from Arde," he said, referring to the rebel organization. But Arde representatives in both Miami and San Jose, Costa Rica, denied that their forces were holding the Americans.

At one time Pastora's forces belonged to a rebel coalition called Arde based in Costa Rica, but the organization broke up when most of its leaders joined forces with the much larger Nicaraguan Democratic Force based in Honduras. Pastora's group now appears to be using the name Arde or Arde Sur.

At noon today, Armstrong radioed that a rebel leader had arrived and was meeting with the Americans and the journalists. According to Armstrong, the leader, who was not identified, insisted that the guerrillas did not belong to Arde, but were Nicaraguans belonging to "an independent anticommunist group."

The guerrilla leader, according to the transcript of Armstrong's message, then allowed them all to get back on their boat and leave.

"They have promised us a peaceful voyage," said Armstrong, according to the transcript. He said the Americans had passed much of their time "singing and praying."

Tinoco said at a press conference that Nicaragua still wanted to establish a demilitarized zone along the border that might be patrolled by forces from neutral nations. Talks between the Costa Rican and Nicaraguan governments were held last fall in Paris, mediated by the French government, but they broke off in October without agreement.