* Investigators hope to identify missing, gather evidence
* Hundreds killed in 1989 riots against free market reform
* Some victims' families say truth will not be established
CARACAS, Sept 21 (Reuters) - Venezuela on Monday began to open unmarked graves of dozens of people killed when police and soldiers crushed riots against free market policies by poor inhabitants of Caracas two decades ago.
Public investigators removed remains from the tombs in one of Caracas's main cemeteries and hope to match them to some of the hundreds of riot victims who died in the outburst of unrest sparked in 1989 by increases in gasoline prices.
The forensic evidence could help bring to justice officials who rights activists say ordered the security forces to open fire to contain the days-long rioting known as the Caracazo.
"Twenty years of impunity must end here, this must set a precedent that soldiers and politicians cannot kill people again," said Jose Luis Martinez, 42, who lost a kidney in the protests. He says he was shot by a policeman simply for being on the street.
Leftist President Hugo Chavez says the riots were a revolt against U.S.-backed economic measures and its repression sowed the seeds for his coming to power a decade later.
The government is preparing to try three high-ranking soldiers, including former Defense Minister Italo del Valle, whom it accuses of ordering the massacre after days of looting in Caracas and other cities.
Officially, 340 people were killed in the turmoil that began on February 27, 1989, but some rights groups say more than 1,000 died.
Some victims' families protested the exhumation because forensic studies are to be carried out by the army without the presence of international experts that would ensure a fair investigation.
"We cannot approve this process because they are violating several basic principles," said Iris Medina, whose husband was killed in the rioting. Medina belongs to rights group Cofavic, founded to seek justice for victims families.
"Firstly, international experts should be here and, secondly, the bodies should not me taken to Fort Tiuna, because the military is to blame," Medina said.
The fort in Caracas is one of Venezuela's main bases. While former soldier Chavez strongly condemns the Caracazo massacre, he may also be wary of upsetting the army too much. In 2002 he was briefly ousted in a military-backed coup.
Latin American countries from Guatemala to Argentina have dug up graves for evidence against military officers accused of mass murders and forced disappearances of people in "dirty wars' against leftists in the 1970s and 1980s. Attempts to establish the truth are often colored by political interests.
Venezuela's Attorney General Luisa Ortega said the investigation was being carried out fairly.
"The Venezuelan state expressly admits its responsibility. Why would we now want to hide anything about the Caracazo? What we want is that everything be transparent and establish who was responsible, whoever it is," Ortega said at the cemetery. (Writing by Frank Jack Daniel, editing by Anthony Boadle)
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