By Mike McDonald

GUATEMALA CITY (Reuters) - Guatemala's highest court on Monday overturned a genocide conviction against former dictator Efrain Rios Montt and reset his trial back to when a dispute broke out a month ago over who should hear the case.

Rios Montt, 86, was found guilty on May 10 of overseeing the killings by the armed forces of at least 1,771 members of the Maya Ixil population during his 1982-83 rule. He was sentenced to 80 years in prison.

However, in a ruling on Monday, the country's Constitutional Court ordered that all the proceedings be voided going back to April 19, when one of the presiding judges suspended the trial because of a dispute with another judge over who should hear it.

It was unclear when the trial might restart.

Rios Montt's conviction was hailed as a landmark for justice in the Central American nation, where as many as 250,000 people were killed in a bloody civil war lasting from 1960 to 1996.

When Rios Montt was in power, his government launched a fierce offensive in which soldiers raped, tortured and killed tens of thousands of Maya villagers suspected of helping Marxist rebels. Thousands more were forced into exile or had to join paramilitary forces fighting the insurgents.

After he was sentenced, a court ordered the government to apologize for atrocities committed against indigenous people.

Ana Caba, an ethnic Ixil who survived the civil war after fleeing her home, was stunned by the Constitutional Court's decision.

"I'm distressed," she told Reuters. "I don't know what's happening. That's how this country is. The powerful people do what they want and we poor and indigenous are devalued. We don't get justice. Justice means nothing for us."

IRREGULARITIES

At the time the row broke out between the judges, a number of appeals were lodged with the Constitutional Court over alleged irregularities in the handling of the case.

One related to Francisco Garcia, one of Rios Montt's defense lawyers, who had just won an appeal to be readmitted to the case. Garcia was thrown out when the trial began for repeatedly trying to have two of the three presiding judges recused.

When Garcia was reinstated, he tried to recuse the judges again, but they rejected his bid and proceeded with the case.

The Constitutional Court said the judges should have suspended the trial until the recusal attempt had been officially resolved. A spokesman for the court could not say how the recusal bid needed to be formally settled.

Diana Cameros, a psychologist who attended the Rios Montt trial, attacked the Constitutional Court over its ruling.

"It's absurd," she told Reuters. "It said in a previous ruling that the process couldn't be wound back to stages that had already concluded, and now it's saying something that contradicts what they said before."

The court said it had given the judges who sentenced Rios Montt 24 hours to comply with its order.

After spending a couple of nights in prison, Rios Montt was transferred to a hospital last week for treatment for respiratory and prostate problems.

He came to power in a bloodless coup on March 23, 1982, and ruled for 17 months during one of the most brutal phases of the conflict until he was toppled in August 1983. He has repeatedly denied the charges against him.

Former U.S. President Ronald Reagan supported Rios Montt's government and said in late 1982 that the dictator was getting a "bum rap" from rights groups for his military campaign against left-wing guerrillas during the Cold War.

Reagan also once called Rios Montt "a man of great personal integrity."

The retired general returned to politics after his fall from power and later unsuccessfully ran for president. For years, he avoided prosecution because he had immunity as a congressman. That ended when he left Congress in 2012.

Until August 2011, when four Guatemalan soldiers received 6,060-year prison sentences for mass killings in the northern village of Dos Erres in 1982, no convictions had been handed down for massacres carried out during the war.

(Reporting by Mike McDonald; Editing by Dave Graham, Kieran Murray and David Brunnstrom)