By TIM WEINER and GINGER THOMPSON
 
MEXICO CITY, July 25 - A Mexican judge on Saturday denied a special prosecutor's request for an arrest warrant on charges against former President Luis Echeverría for the killings of student protesters in 1971, a stinging setback in the pursuit of justice for the abuses of the authoritarian government that ruled this country for most of the last century.

The decision amounts to a temporary dismissal of the case filed Friday against Mr. Echeverría, president of Mexico from 1970 to 1976.

The special prosecutor, Ignacio Carrillo Prieto, said he would appeal the decision and would ask Mexico's attorney general to take the case to the Supreme Court.

Mr. Carrillo Prieto said the judge did not "adequately weigh each and every one of the elements" of the 9,385 pages of evidence and testimony he had presented Friday. The prosecutor did not explain why the judge had thrown out the charges, and there was no statement by the judge, who was appointed by Mexico's Supreme Court.

The charges accused Mr. Echeverría of genocide for his role in controlling a government hit squad that attacked demonstrators in Mexico City on June 10, 1971. About 25 marchers died that day. President Echeverría denied that anyone had been killed.

Mr. Echeverría, 82, is the elder statesman of the Institutional Revolutionary Party, or PRI, which ruled Mexico from 1929 until 2000, when Vicente Fox won the presidency. No former president has ever been charged with a crime in Mexico.

President Fox appointed the special prosecutor more than two years ago, with great fanfare. He said the prosecutor would uncover secrets that had shrouded the government's war against the Mexican left from the late 1960's to the early 1980's, which killed hundreds and left at least 275 missing and presumed dead.

"In Mexico, there has never been a genocide," Juan Velasquez, Mr. Echeverría's lawyer, said in an interview. "There have been clashes, deaths, and massacres, but never genocide, as a policy of the state."

"In my 35 years in as a lawyer, in all my political cases, this one is the easiest of them all," said Mr. Velasquez, who has defended a former Mexican president and another former president's brother in civil matters. "It is doomed to fail."

The prosecutor's charge of genocide - in Mexico, "systematic crimes against the lives of members of any national group," including political dissidents, appears for the moment to have misfired.

"The irony is by that charging Echeverría with genocide, he may have ensured impunity," said George Vickers, Latin America director at the Open Society Institute in Washington. "The justice system in Mexico has a theatrical dimension to it, and the performance of the special prosecutor is increasingly looking like tragedy disguised as farce."

Rights groups, and some former members of the special prosecutor's office, have said for months that the office was faltering, running into a stone wall built by the Mexican military and the former ruling party. Under intense pressure from that party, Mr. Fox did not establish a "truth commission" to investigate the past, as other nations have done.

The former ruling party, which still holds a plurality in the Mexican Congress, has warned the president against a witch hunt for political gain. Now Mr. Fox has distanced himself from the issue, saying he wants to uphold the independence of Mexico's judicial system. In his weekly radio address to the nation on Saturday, he did not mention the Echeverría case.