LA PAZ, June 20 (Reuters) - Bolivian officials on Friday criticized a prosecutor's decision to release two men whom they believe had planned to assassinate leftist President Evo Morales.

Police said the men were detained on Thursday at an airport in the opposition bastion of Santa Cruz shortly before the Bolivian leader arrived there. One of the men was carrying a rifle and ammunition, said Cabinet Chief Juan Ramon Quintana.

A prosector ordered the men freed hours after their arrest, saying there was a lack of evidence -- a decision that Quintana slammed.

"We are denouncing to the country and to the world that behind this assassination attempt there is a plot, a sinister plan whose reach we still don't know. But we are aware of the motives," he told reporters.

Many in Santa Cruz, the country's wealthiest province, stiffly oppose Morales' reforms, which include redistributing land to the poor and overhauling the constitution.

The eastern region's conservative leaders are waging a campaign for greater autonomy from the central government.

Last month, Santa Cruz residents voted overwhelmingly in a referendum for the regional government to take greater control over local judicial and economic matters.

Government officials said Morales, Bolivia's first indigenous president, continued his visit to the area despite the arrests. (Reporting by Ana Maria Fabbri; Writing by Kevin Gray; Editing by Jackie Frank)

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