Living in Washington D.C. gives me the opportunity to attend many protests most notably the protest against immigration laws enacted to deport illegal aliens. So how does one take photos without requiring model releases from the crowd? The best way is to shoot from behind so that no one is recognizable. Otherwise you can shoot just one person from the front and try to get a release. Some photographers just shoot the protest sign eliminating the crowd by pointing their camera upwards. Be sure to clone out any logos on t-shirts and hats if shooting a crowd from behind and always try to get as many signs as you can in the photo.
Other ongoing protests in Washington D.C. are anti war protests against the war in Iraq and protests by the Falun Dafa society against forced human organ donation in China. Pay attention to the news for dates and places. Generally speaking most protests in Washington D.C. take place on the mall, at the Capitol building and the White House. Other cities like Los Angeles, Chicago and New York City have protests as well. Good luck and good shooting!