INDIO, Calif. - Sean Penn spoke at the Coachella Valley Music and Arts Festival on Sunday, urging the young crowd to involve themselves politically.

The Oscar-winning actor, a late addition to the music festival, joking referred to his out-of-place billing among the 125-plus performers.

Wearing a T-shirt and jeans and smoking a cigarette while he sat on a stool, Penn said he unfortunately couldn't perform his "a cappella Celine Dion cover act" since he had "compromised his upper register."

Instead, Penn urged festival-goers to join him on his "Dirty Hands Caravan," a biodiesel cross-country bus trip he plans to launch Monday, arriving in New Orleans on May 4. The purpose of the trip, which he hopes 300 will join, is to encourage young people to be more politically and environmentally involved.

"The government can't do it," Penn said. "They can't save this thing."

Penn said that while younger generations were smarter and more technologically savvy than any before it, they were separating themselves through technology.

He also criticized the war in Iraq.

"For the 3,000 people we lost on 9/11, we've lost 4,000 in this war, and that's just American soldiers," Penn said.

"And why did we let it happen?" he added. "It's simple: We let it happen."

The "most powerful third party is you and me," Penn said.

Penn was one of the few participants to discuss politics at the Southern California festival, where dancing and music were far more prevalent