Sean Penn has expressed his admiration and support for Venezuelan President Hugo Chavez at a candlelight vigil in Bolivia.

Chavez is in Cuba for an operation to treat an unspecified cancer.

Penn is visiting Bolivia to support jailed American Jacob Ostreicher who is accused of corruption and continues a high-profile battle with Bolivian officials.

Penn grew emotional as he talked about the ailing leader.

"He is one of the most important forces we have had on this planet," Penn told the crowd.

"I wish him nothing but that great strength he has shown over and over again," he said.

Chavez will have a third session of cancer surgery in Havana in the coming days after announcing on Saturday that tests had found the return of "some malignant cells" in the area where tumours were removed last February.

He took the precaution of naming Venezuela's Vice President Nicolas Maduro, as nominal successor if the operation goes wrong.

Another message of support came from Peru's president Ollanta Humala.