"I do this out of a sense of participating in democracy," said Kramer, who said he plans to vote for Democrat Barack Obama in November. "Democracy requires participation. It's not just a theory."
All the artists donated their time to perform in Denver, event publicist Carrie Lombardi said.
"It is a peaceful protest through music," she said.
Rage Against the Machine also has a concert scheduled Sept. 3 in Minneapolis during the Republican National Convention in the adjacent city of St. Paul.
The Denver festival is hosted by Tent State University and Iraq Veterans Against the War. Organizers say about 8,000 tickets will be given away free by lottery.
Lottery entries will be accepted 11 a.m.-6:30 p.m. Aug. 24-26 at downtown Denver's Cuernavaca Park, Tent State University's base for the convention. Winners will be notified the evening of Aug. 26.
The Democratic National Convention will be Aug. 25-28.
Tent State University describes itself as a college campus-based movement aimed at ending the Iraq war.
The group originally sought to camp at Denver's 314-acre City Park during the convention, but the city said a park curfew would prevent anyone from staying overnight.
Tent State then decided to move to Cuernavaca Park, which is smaller but closer to the Pepsi Center, where most of the convention will be held.
Kramer said he and Tom Morello of Rage Against the Machine have long worked together on music and activism.
"When the opportunity came up to take our electric guitars and make some unholy noise to try to stop a war, it sounded familiar to me," Kramer said. "It's something we did 40 years ago, as part of a generation that said the war in Vietnam was wrong and needed to stop. We're part of a generation saying the war in Iraq is wrong and needs to stop."
