American commanders said their forces had killed 1,200 insurgents in that offensive, while taking more than 500 American casualties. But insurgent groups said later that many of their fighters had left the city for Ramadi, Mosul and other insurgent strongholds before the American assault.

Under a pledge to rebuild the city and compensate those who lost their homes, the Americans have spent about $100 million. But insurgents who never left under the American bombardment, or who infiltrated back through the tight cordon that American and Iraqi troops have thrown around the city, have kept up a steady stream of attacks, including suicide bombings, roadside explosions and assassinations of Iraqi government officials and others who have drawn the insurgents' wrath.

Earlier this week, two of Falluja's leading Muslim clerics, Hamza Abbas al-Issawi, 70, considered the city's grand imam, and Ayad al-Izzi, both of whom had urged Sunni Arabs to defy the insurgents and vote in the Dec. 15 elections for a full four-year national government, were shot and killed. Both had received insurgent death threats in recent months.

Tensions appeared to be rising ahead of the election, when American and Iraqi officials are hoping for a repeat of the October constitutional referendum, when 170,000 votes were cast in Falluja, the strongest turnout of any Sunni Arab area in Iraq. Iraqi election officials calculated that 80 percent of the votes were against the constitution, but celebrated the fact that the city had chosen to take part in the political process.

In Falluja's mosques, angry residents have vowed in recent days to avenge the clerics' killings by hunting down Islamic extremists loyal to Abu Musab al-Zarqawi, America's most-wanted man in Iraq. The anger spread on Friday to fiery condemnations at the main weekly prayers at two of Baghdad's most militant Sunni Arab mosques.

At the Mother of All Battles Mosque in the west of the city, the preacher, Sheik Ali Abu Hassan, called the killers "murderers" and said believers should respond by voting in large numbers. At the Abu Hanifa Mosque in the eastern Adhamiya district, a stronghold of support for Iraq's ousted ruler, Saddam Hussein, the preacher, Sheik Ahmad al-Samarrai, said, "The election is both legitimate and necessary, and your duty to vote is heavier than a mountain."

In another threat of violence by the insurgents, a new videotape broadcast by Al Jazeera satellite television on Friday evening threatened to kill four Western Christian peace activists kidnapped in Baghdad last Saturday, one of them an American, unless all insurgent prisoners held in American and Iraqi detention centers are released.