But police are now guarding the site in Kampot province from further pillaging by the villagers, said Khieu Sokhon, a provincial district police chief.
He said the site has several small graves holding remains of Cambodians who died during the Khmer Rouge's 1975-79 rule.
He said a group of Vietnamese soldiers on Saturday excavated the site to look for remains of their fellow soldiers who died during their occupation of Cambodia in the early 1980s but found only bones of Cambodians.
After the Vietnamese soldiers left, Cambodian villagers, armed with hoes and spades, flocked to dig up the graves because they thought valuable jewelry was among the buried bones, he said.
He said the villagers unearthed some 120 bodies and found just several earrings.
"We have now deployed police at the site to prevent people from digging it up," Khieu Sokhon said.
Digging Khmer Rouge-era graves for valuables was common after the now-defunct communist movement was overthrown by Vietnamese troops in 1979.
Khmer Rouge implemented radical policies that led to the death of some 1.7 million people from starvation, disease, overwork and execution. Those who perished were mostly buried in mass graves.
Kampot province is about 130 kilometers (80 miles) southwest of the capital Phnom Penh.