The Mystic River star was photographed holding a national flag in Cairo as he joined a protest urging military leaders to hasten the transfer of power to civilians following weeks of unrest that led to Mubarak's resignation earlier this year (11).
Egyptian actor Khaled el-Nabawi told reporters he invited Penn to visit Egypt, hoping his presence would help to show the world that the country is a safe place to visit again following the 2011 uprising.
Penn recently hit the headlines when he was linked to the release of two American hikers who were charged with spying and jailed in Iran.
Reports suggested the actor urged his friend, Venezuelan leader Hugo Chavez to speak to Iran's President Mahmoud Ahmadinejad about the plight of the hikers. Chavez is an ally of the Iranian leader.
Last week (23Sep11), Venezuelan state media representatives reported that Chavez was part of the mediation process that resulted in the release of the hikers.
Penn is also very active in the recovery of earthquake-ravaged Haiti, where he has co-founded a charity organisation aimed at funding relief, healthcare and rebuilding programmes in the Caribbean following last year's (10) disaster.