"With this ... I underscore my commitment to the promotion and defense of human rights and the strengthening of public institutions," Fox said in a statement at the Los Pinos presidential headquarters.

Ascencio and Guzman were sentenced to 20 years in jail for their role in protests by Lake Patzcuaro fishermen in which officials from the government's environmental attorney general's office were briefly held hostage after attempting to seize fishing nets. Local human rights groups considered the two fishermen political prisoners.

It was the second group of pardons issued by Fox, who took office in December 2000. The cases stemmed from arrests made during the prior administration of former President Ernesto Zedillo.

In November 2001 Fox freed peasant ecologists Rodolfo Montiel and Teodoro Cabrera after they spent more than two years in jail on weapons and drugs charges, which human rights groups say were fabricated by soldiers who arrested them.

Despite the pardons, rights groups have criticized the president for doing too little to end human rights abuses deeply rooted during the Instutional Revolutionary Party's (PRI) 71-year rule, which ended when Fox took office.

In January the president appointed a special prosecutor to investigate atrocities from Mexico's 1970's "dirty war" against leftists. The prosecutor has vowed to pursue any army officers or politicians involved in torture or murder, despite opposition from entrenched interests.