Echeverria would have been the first president to face criminal charges

A Mexican judge refused to order an arrest warrant for Luis Echeverria, who has been accused of ordering a massacre of students in 1971.

Mr Echeverria, 82, ruled Mexico from 1970 to 1976.

He had faced the prospect of becoming the first Mexican president in modern times to face criminal charges.

Notorious event

Prosecutors said "dozens" of people died on 10 June 1971 at the hands of the Halcones (Falcons) - a paramilitary force allegedly created by Mr Echeverria's ruling party.

It was a notorious chapter in the "dirty war" between government forces and leftist guerrillas that took place in the 1960s and 1970s.

But Judge Cesar Flores threw out the case against Mr Echeverria, without releasing the reasons for the decision.

Human rights groups and families of the victims' were despondent.

"We don't have any hope," said Rosario Ibarra, whose son disappeared in 1975.

"Charges come and go, arrest warrants come and go, they get injunctions, and in the end nothing is ever done."

The head of one human rights organisation told the BBC: "This is a very unfortunate decision for the history of Mexico; the judge must have been very frightened."

Truth commission

Special prosecutor Ignacio Carrillo said he would appeal against the judge's decision in the next three days.

But in some quarters prosecutors were criticised for bringing the case at all.

Critics said the genocide count was only chosen because it was one of the few charges not to have expired under the 30-year statute of limitations.

A former adviser to President Vicente Fox, Adolfo Aguilar Zinser, said:

"This was the wrong way to do this... The question is whether genocide was committed on Corpus Christi, or whether they did not have anything else to charge them with, so they chose a crime that might not fit the events."

The BBC's Claire Marshall, in Mexico City, says there have been calls for the "dirty war" to be examined by a truth and reconciliation commission, rather than the courts.