The Mexican government sought yesterday to charge former president Luis Echeverria with genocide in connection with a massacre in 1971 in which security forces killed at least 30 young protesters in Mexico City, officials said. The pending arrest breaks with Mexico's long history of impunity for its political leaders and marks the first time a former president has faced the possibility of criminal charges.

Special prosecutor Ignacio Carrillo Prieto asked a judge late Thursday to issue an arrest warrant for the 82-year-old former president, who governed from 1970 to 1976 during the most violent period of Mexico's campaign against anti-government activists, known as the "dirty war." Mexican news reports said Judge Jose Cesar Flores was expected to issue a warrant for the arrest of Echeverria and several former aides by today.

Officials in Carrillo Prieto's office, who declined to comment yesterday, have not publicly explained the legal basis for the genocide charge. Echeverria's legal advisers have argued that there is insufficient evidence for such a charge. David Penchyna, secretary of the national council of the Institutional Revolutionary Party, or PRI, which ruled Mexico for 71 years until the election of President Vicente Fox in 2000, called the allegation of genocide "totally absurd" and accused the Fox administration of "politicizing justice" and seeking only to "annihilate its political enemies."

Echeverria's attorney, Juan Velazquez, told a television interviewer in Mexico City that the former president was ready to comply with the judge's ruling. Analysts said they considered it unlikely that Echeverria would be jailed, citing a law recently passed by Congress that allows judges to order house arrest rather than jail for defendants older than 70. The judge would have 72 hours after Echeverria's arrest to decide whether to issue formal charges.

The action against Echeverria marked an extraordinary departure for Mexico, where presidents from the PRI ruled with what amounted to dictatorial authority. Seeking charges against a former president was long considered politically impossible, and the prosecutor's decision to request Echeverria's arrest dominated the news in Mexico yesterday.

"This is a historic event without precedent in Mexico," said federal congressman Salvador Martinez Della Rocca, a former student activist who was present at the 1971 massacre and escaped when the shooting started. "It's important that these events go down in the history books, that the youth know the true history of their country, that we fought for our freedoms and the answer was a massacre."

Sergio Aguayo, a leading human rights activist in Mexico City, said the action against Echeverria broke "a very long Mexican tradition of impunity for presidents and powerful people." He said the potential genocide charges against Echeverria, similar to those that have been pursued against Gen. Augusto Pinochet, the former Chilean dictator, showed that Mexico was evolving into a more democratic nation that would no longer tolerate abuses by its leaders. "What is important is the message that it sends that nobody is above the law," Aguayo said.

The move to bring charges against Echeverria, former internal security minister Mario Moya and former attorney general Julio Sanchez Vargas is a milestone in Mexico's efforts to investigate state crimes committed during the dirty war era, which extended from the 1960s to the 1980s. Fox campaigned on a pledge to shed light on the abuses of the past. Shortly after he took office, he acknowledged that the government had been responsible for the disappearances and killings of hundreds of activists.

He appointed Carrillo Prieto to investigate, but many critics doubted that the former law professor would have the political backing, power or will to take on the high-ranking PRI leaders who were widely believed to have directed the violence against activists. While some of the activists were members of radical guerrilla groups, many were students, teachers and farmers fed up with the PRI government's authoritarian policies.

"This is a fulfillment of Fox's campaign promise to somehow settle scores with the past," said Jorge G. Castaneda, Fox's former foreign minister.

The charges against Echeverria could also pose a dilemma for the Mexican political and judicial systems, several analysts said yesterday. They predicted that the accusation would deepen the rift between Fox and the PRI, which still controls Congress. They also said genocide was a difficult charge to prove, and failure could precipitate what Adolfo Aguilar Zinser, Fox's former national security adviser and former ambassador to the United Nations, called a "deep crisis of credibility."

Aguilar Zinser said many Mexicans would assume that the judicial system had been compromised by corruption if Echeverria was not tried and convicted. "If the basis of the prosecution is not firm, it could be the beginning of a horrible time," he said. "It could lead to a lot of political revenge." He added that "the moral judgment has already been made" by the public about Echeverria's guilt.

For many Mexicans, Echeverria symbolizes the worst of the government's past abuses. He was interior minister, the second-most-powerful post in the government, on Oct. 2, 1968, when policemen and soldiers opened fire on anti-government protesters and killed an estimated 300 people in Mexico City's Tlatelolco Plaza -- the most infamous incident of the dirty war era. He was president on June 10, 1971, when government-backed security forces opened fire on student demonstrators during a march in Mexico City, killing at least 30 people in what became known as the Corpus Christi massacre.

In international legal rulings, a finding of genocide has generally been required to meet the U.N. Convention definition of an act "committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group." The Mexican penal code's definition approximates that characterization.

Eric Olson, a spokesman for Amnesty International in Washington, said he doubted that the allegations against Echeverria constituted genocide, but he noted that the prosecution's case was unknown.

"This is uncharted territory in Mexico, but I think the prosecutor has taken a bold step," Olson said. "This could help clear up a real tragedy in Mexico's past and lend some credibility to Mexican institutions that are bankrupt of any credibility at this time. And I think there is a glimmer of hope that Mexican prosecutors and judges will do the right thing."

People interviewed on the street yesterday in Mexico expressed skepticism that Echeverria would be punished.

"Nothing's going to come of it, it's much ado about nothing," said Laura Dominguez, 61, a clothing designer from Sinaloa state. "I wish all the presidents would be charged for what they've done to this country. They've sunk the country into misery."

Researchers Gabriela Martinez in Mexico City and Bart Beeson in San Miguel de Allende, Mexico, contributed to this report.

Luis Echeverria may face genocide charges for a massacre in 1971. Luis Tunon, from a Mexican group seeking redress for political crimes, holds a banner demanding the conviction of former president Luis Echeverria for killings during the "dirty war."Special Prosecutor Ignacio Carrillo Prieto sought Echeverria's arrest in connection with a 1971 massacre of 30 protesters but declined to explain the legal basis for charging him with genocide.