BAGHDAD — Iraq's Sunni Arabs, voting en masse for the first time since the overthrow of Saddam Hussein, turned out in large numbers Saturday for a nationwide constitutional referendum. Insurgents largely halted attacks, giving Iraq's capital and much of the rest of the country a rare day of peace that belied deep fractures exposed by the referendum.

Turnout reached 93 percent in the heavily Sunni western city of Fallujah after clerics and others went door-to-door telling residents it was safe enough to venture out of their homes, election officials said.

But in some other western cities, fear crushed the potential that had been suggested by heavy Sunni voter registration. In Ramadi, election day opened with automatic-weapons fire around at least one polling site. There were sporadic explosions as U.S. Marines were in the streets. Turnout there was 10 percent. "People are terrified and don't want to risk their lives," said an electoral official, Nadhum Ali.

The strong overall turnout in the west, however, raised the possibility that the disempowered Sunni minority could defeat the draft charter, which endorses a loose federal system with a weak, religiously influenced central government. Many Sunnis fear the draft would bring the breakup of Iraq into ethnic and religious substates and make permanent their loss of power to the Shiite majority after the toppling of Saddam.

Defeating the charter would take a two-thirds no vote in at least three of Iraq's 18 provinces. Turnout was strong in three heavily Sunni provinces that had been expected to vote against it: Salahuddin, with 75 percent turnout reported by the local electoral director; Diyala, with 65 percent turnout; and Anbar, whose provincial total was not released Saturday.

First returns are expected today; final, unofficial results are due Thursday. A close vote would risk heightening Sunni suspicions about the political process.

In his weekly radio address, President Bush said Saturday that the referendum dealt "a severe blow to the terrorists" while sending a message to the world. "Iraqis will decide the future of their country through peaceful elections, not violent insurgency."

Bush said the referendum was "a critical step forward in Iraq's march toward democracy." Despite eroding public support for the war, Bush also promised to stay the course in Iraq. "America will not run, and we will not forget our responsibilities," he said.