By Eduardo Garcia

LA PAZ, Dec 3 (Reuters) - A regional commission investigating the killings of at least 20 Bolivians during anti-government protests in September described the incident as a massacre on Wednesday.

The Unasur group of South American nations vowed to probe the killings, which took place in an Amazon province as anti-government protesters attacked natural gas pipelines and stormed public buildings.

The unrest flared during a bitter power struggle between leftist President Evo Morales and conservative rivals opposed to his drive to implement a new constitution.

Unveiling his findings on behalf of the 12-member panel, commission head Rodolfo Mattarollo said some of the 20 mostly Morales supporters killed in Pando province were murdered.

Mattarollo, an Argentine lawyer and Human Rights expert, said that some of the killers worked for Pando's opposition-controlled provincial government.

"On Sept. 11 2008, in the village of Porvenir and other places in the Pando province, a massacre occurred," he said, presenting the commission's report in the presidential palace.

As the turmoil raged in the sparsely-populated region near the Brazilian border, several South American leaders issued a statement of support for Morales and warned his opponents not to stir unrest in the natural-gas rich Andean nation.

Morales, who called the protests an attempt to destabilize his government, welcomed the commission's findings.

"It wasn't a clash, it was a massacre ... Conspiring against democracy, against people's lives, using terrorism ... it's not the best way," Morales said.

His government declared martial law in Pando and arrested provincial Gov. Leopoldo Fernandez, accusing him of inspiring the violence. It lifted the state of siege late last month. (Editing by Alan Elsner)

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