American flags were set on fire Friday to chants of "no, no for occupation" as followers of an anti-U.S. Shiite cleric marked the sixth anniversary of the Iraq war.

In Baghdad and five other Iraqi cities, supporters of Moktada al-Sadr either marched or stood in protest after prayers to demand the release of their allies detained at Iraqi and U.S.-run prisons.

The protests came as a suicide bomber in Fallujah killed an Iraqi police officer and five other people, including civilians, in an attempted attack on the home of the local leader of Sunni security volunteers who turned against Al Qaeda.

Also, a pair of roadside bombs exploded within 10 minutes of one another after sundown Friday, wounding four policemen and three civilians in Baghdad's Karradah district, police said. A police colonel and his aide were wounded in a bombing Friday in Saddam Hussein's hometown of Tikrit, police said.

In the capital, an aide to Sadr, Sheik Haidar al-Jabiri urged supporters to join an April 9 march to protest the six-year anniversary of Americans taking over the city.