The Oscar winner made a surprise appearance at the gathering in La Paz, which was held as Chavez undergoes his fourth cancer surgery in Cuba.
Taking the stage in a Venezuelan flag T-shirt, Penn told the crowd, ""He's one of the most important forces we've had in this planet. And I'll wish him nothing but that great strength he has shown over and over again. I do it in love, and I do it in gratitude.
""I just want to say, from my very American point of view, of my friend President Chavez: it is only possible to be so inspiring as he is, as a two-way street. And he would say that his inspiration is the people.""
The Oscar winner has not been shy in voicing his support for Chavez's government, despite the fact the President is not considered an ally to the U.S.
Penn is also in Bolivia this week campaigning for for the release of incarcerated businessman Jacob Ostreicher, who has spent 18 months behind bars for money laundering. Ostreicher's supporters are upset there was no trial before the businessman was jailed.