TUXLA GUTIERREZ, Mexico (AP) _ The sister of a Roman Catholic bishop mediating peace efforts in southern Mexico was in serious condition in a hospital Friday after being attacked with a hammer in the bishop's cathedral.

Bishop Samuel Ruiz, his face grim, linked the attack to an apparent ambush three days earlier on his car and to broader violence he claimed was fanned by the Mexican government.

Ruiz, whose activism on behalf of the state's large Indian population has aroused fierce opposition from conservative elements, is a leading mediator in peace talks between the government and a leftist rebel movement.

Maria de la Luz Ruiz Garcia was beaten Thursday afternoon in San Cristobal de las Casas, 35 miles east of Tuxla Gutierrez, by a man Chiapas state police identified as her godson, Miguel Mendez Toporek.

Dr. Gustavo Lazos Constantino said she had suffered three skull fractures and ``her life is in danger.''

Her son, David Murphy, said the attacker seemed to be ``talking cordially'' with her in the cathedral when he pulled out a hammer and hit her.

Ruiz, after visiting his sister, did little to clarify the motive. ``The boy is a victim of misinformation,'' he said, referring to the attacker. He did not elaborate.

Mendez was arrested and was being held in jail Friday.

Ruiz, meanwhile, complained to reporters about Mexico's ``framework'' of violence.

``For a long time many people have been affected, by killings, by expulsions, with burnings of houses, with differences created by the army or by public security without there being an adequate correction,'' he said.

About 300 people have been killed in clashes involving politics and land issues in the southern state of Chiapas since a cease-fire in January 1994 froze a brief revolt by the Zapatista National Liberation Army.

Ruiz said on Oct. 24, members of a group called Peace and Justice, linked to the governing Institutional Revolutionary Party, had tortured three members of the ruling party by mistake in the town of Tumbala.

On Tuesday, gunmen fired on a motorcade carrying Ruiz and fellow Bishop Raul Vera as they traveled through a rural village in Chiapas. Three other people in the caravan were wounded.

Members of the caravan told reporters they recognized members of the radical Peace and Justice group among the attackers.

On Wednesday, Chiapas Gov. Julio Cesar Ruiz Ferro declared his support for Ruiz's work as a mediator in stalled peace talks between the government and Zapatista rebels, and ordered an investigation into Tuesday's attack.

But the bishop, a strong supporter of Indian rights, was unimpressed.

``There is a growing impunity and there are things that are distorted, disguised and promoted,'' he said. ``We can't reach peace with an attempt on my life.''

Ruiz has received repeated threats from people who blame him for fomenting the Indian rebellion with his social teachings.