MEXICO CITY, AUG. 28 -- The head of Nicaragua's nongovernmental human rights commission and the president of the Nicaraguan Bar Association have begun a hunger strike to protest their detention by Sandinista police and a cutoff of family visits, according to Nicaraguan human rights activists.

Lino Hernandez, the director of the Permanent Commission on Human Rights, and Alberto Saborio, president of the independent bar association and secretary general of the opposition Conservative Party, were arrested Aug. 15 when police armed with clubs, dogs and electric cattle prods broke up a demonstration by the Democratic Coordinating Council, an opposition umbrella group.

The two were summarily sentenced to 30 days in jail on charges of "disturbing public order," but were receiving daily visits and deliveries of food from their families until Tuesday, when these were suddenly cut off, the Nicaraguan sources said by telephone from Managua.

The case of Hernandez and Saborio has attracted attention from foreign human rights organizations and aroused speculation about differences within the Sandinista government over a Central American peace plan signed by President Daniel Ortega Aug. 7 in Guatemala City.

Specifically, the arrests have raised questions about whether Interior Minister Tomas Borge, who controls the Sandinista security forces, and other comandantes considered hard-liners fully support the peace plan -- which commits the Managua government to potentially far-reaching democratic changes.

Last week, Amnesty International put out an "urgent action appeal" expressing concern about the summary nature of the two men's sentence and urging their immediate release unless formally charged with a recognizable criminal offense, a spokeswoman for the organization's Washington office said.

Another human rights group, Americas Watch, sent a telex to the Nicaraguan Foreign Ministry Aug. 17 asking for the release of Hernandez and saying that he should not be detained for his rights work or involvement in a peaceful demonstration, a spokeswoman said.

The American Bar Association and four U.S. human rights groups, including the Puebla Institute and the International League for Human Rights, also issued a joint statement protesting the jailing of Hernandez and Saborio.

According to the human rights commission in Managua, Sandinista authorities permitted Americas Watch Vice Chairman Aryeh Neier to visit Hernandez Wednesday, but have refused since Tuesday to allow Hernandez's wife or commission members to see him. Hernandez and Saborio are being held in a police substation.

In a statement smuggled out of jail Wednesday, the two said they were beginning a hunger strike to protest their "unjust and arbitrary detention," which they said was "a political reprisal for our work on human rights." They vowed to maintain the hunger strike until their release.

According to Nina Shea of the Puebla Institute, the arrests showed "a contemptuous attitude toward the rule of law." She said that given the positions of Hernandez and Saborio as heads of a human rights commission and an independent bar association, they are currently "the only ones in the hemisphere of that stature in prison."

According to witnesses, Hernandez was arrested as he was trying to leave an opposition demonstration, was accosted by a member of a pro-Sandinista mob who tried to beat him, then was struck by a club-wielding Sandinista policeman.