Sunni Arab voters turned out in force for Iraq's constitutional referendum Saturday as insurgents largely suspended attacks, granting Sunnis a chance to try to defeat the U.S.-backed charter and giving much of the country a rare day of peace that belied the deep fractures exposed by the vote. Voting en masse for the first time since the overthrow of Saddam Hussein, Sunni Arabs cast ballots in large numbers, according to electoral officials and witnesses. Turnout in areas populated by the country's Shiite majority and ethnic Kurds, whose political leaders drafted the proposed constitution, was described by officials as low.

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 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 patrolled 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 Muslim majority after the toppling of Hussein.

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

In his weekly radio address Saturday, President Bush said 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.

Speaking in the Democratic radio address, retired Army Gen. Wesley K. Clark called the vote "an important step toward a democratic Iraq." Still, he said, "let's not kid ourselves about the difficulties that lie ahead." Defeating the insurgency, winning the support of alienated Sunnis, training Iraqi forces and rebuilding the country's infrastructure and economy remain formidable tasks, he said.

For some Sunnis, a vote against the constitution was in part a statement of rejection of the U.S. occupation. But for most, the vote appeared to signal their entry to Iraq's political process in hope of regaining some control over their future -- if not by defeating the charter, then by winning seats in December's parliamentary elections and working to amend it.

If the Shiites consolidate their power, "we will never see stability in Iraq again," said Khalaf Ahmed Khalif, 53, a farmer in Ishaqi, an area of lush farmland amid arid desert in Salahuddin province. "If you think it's bad, just think about double the number of forces the Americans have in the country right now."

In Ishaqi, 300 people voted in January in elections for a transitional government. By late Saturday afternoon, 9,350 had turned out.

The turnout was also high in several larger cities across the mostly Sunni province, including places where the U.S. military has waged a nearly continuous battle against insurgents. Samarra, which reported 35,636 voters by mid-afternoon, had run out of ballots and was requesting more from U.S. commanders.

Not all Sunnis voted "no." In Baghdad, some said they voted for the charter because of security concerns, hoping that the next step would be rule of law.

"I insisted on voting, even though my neighbors told me it would be dangerous," Haifa Ahmed Satoor, 38, a government worker and a Sunni, said in Baghdad before voting "yes."

"I don't want more people killed in the name of Sunni resistance," Satoor said. "We already lost neighbors -- I don't want to lose relatives."

In Sadr City, a vast Shiite area in northeast Baghdad, Mayyada Ahmed already had. She came to a polling center to vote for the draft on the final day of mourning for a cousin shot by Americans at a checkpoint. A few weeks ago, three other male relatives were abducted from their home and later found dead in heaps of garbage.

"We came because we hope the future will be better," Ahmed said, reflexively waving her ink-stained finger in a now-worn symbol of hope from the last election. "We are hoping it will provide safety. We will keep voting until it does."

U.S. troops in Baghdad and most of the country yielded election security to Iraqi forces, save for American convoys that rumbled through Baghdad. As in January, a one-day ban on private vehicles helped block suicide attacks. Iraqi troops manning checkpoints fell into impromptu soccer games with children, who poured into the streets, roller-skating and biking through the city on Baghdad's most peaceful day in months.

Nationwide, security was a "resounding success," with all of the 13 recorded attacks aimed at election targets failing, Lt. Col. Steve Boylan, a spokesman for the U.S. military, said in Baghdad. Boylan said Iraqi forces in the Shiite city of Hilla stopped two women -- one from Jordan, the other from Saudi Arabia -- wearing explosive vests.

One of at least two mortar rounds heard Saturday in Baghdad landed inside the Green Zone, where some lawmakers and others voted behind concrete barriers and concertina wire.

In insurgent strongholds, such as Baghdad's southern neighborhood of Doura, where gunmen sometimes take to the streets by the score, the absence of major attacks suggested the Iraqi militant groups had kept their promise of election-day calm. "There was a large turnout," said lawyer Abdul Amir Yousuf, 68, ringed by Iraqi police.

Coincidentally or not, the vote fell on the date of Hussein's last show election, in 2002, when he declared that his government had been approved by 100 percent of Iraqi voters.

January's elections for national and provincial parliaments were the country's first free votes in almost half a century. But the resulting seating of a Shiite-led government April 28 was the cue for what have been unceasing daily bombings and other political violence, killing thousands.

On Saturday, most of the elation seen in January's vote was gone. Men, leading wives, sisters and daughters to protect them, made straight lines through milling children to the polls and then retreated back inside their homes.

Many voters objected strongly to the last-minute dealmaking that kept many Iraqis from seeing the draft constitution in its final form.

"I may be in the minority, but first of all I am not voting to support something I have not seen, and they never showed us the constitution," said Majeed Khadhan, 35, a taxi driver in Najaf. "Second, the Americans say this is about democracy, but we do not live in a democracy when we are occupied. . . . This constitution will not help that. It was made to please the politicians, not to please the people."

Some, however, retained the excitement they had in January.

"I cannot read, but my sons and daughters read parts of the constitution to me and told me it would provide security and stability in the future," said Mardhiya Omar Ibrahim, 85, who was carried to a polling place in Mosul on her son's back. "So I went out to vote for it because I want the future to be safe and peaceful for my sons and grandchildren."

"I vote a thousand times 'yes,' not only once, because I have not forgotten the mass graves and the torture and the killings," said Abdul Hussein Ahmed, 63, a laborer who emerged from a polling station in Najaf with purple finger aloft.

"Five members of my family were killed by Saddam and his people. But now, with this constitution, everyone is equal under the law," Ahmed said.

In the north, where passage of the constitution with its federal system would enshrine the de facto autonomy Kurds there have enjoyed since the Persian Gulf War, Salih Saeed Mohammed, 64, blew on his ink-stained finger as he left a polling place. "This process legitimizes the fate of our nation," he said, referring not to Iraq but to the Kurdish north, where many believe its establishment as a federal region will lead to independence.

When polls closed in the city of Sulaymaniyah, thousands of people poured into the streets among drivers pounding car horns and waving Kurdish flags.

Correspondents Jonathan Finer in Najaf, Steve Fainaru in Ishaqi and Jackie Spinner in Sulaymaniyah, staff writer Michael A. Fletcher in Washington and special correspondents Bassam Sebti and K.I. Ibrahim in Baghdad, Naseer Nouri and Saad Sarhan in Najaf, Dlovan Brwani in Mosul and Salih Saif Aldin in Tikrit contributed to this report.

Iraqis in the northern city of Mosul wait in long lines to vote in the referendum on the proposed constitution, drafted largely by Shiite and Kurdish leaders. A voter is searched outside a polling station in Baghdad. Security was tight for the nationwide balloting. A man prepares to cast his ballot in Baqubah, a mixed Sunni-Shiite town northeast of Baghdad.