Just ahead of the first free balloting in Iraq in half a century, the nation battened down for the vote, imposing a 7 p.m.-6 a.m. curfew on Friday and closing Baghdad International Airport (search). Five U.S. soldiers were killed in the capital and insurgents blasted polling stations across the country.

The curfew will remain in effect through Monday and the nation's borders will be sealed for the election period. Medical teams are on alert and nationwide restrictions on traffic will be imposed from Saturday to Monday to try and deter car bombs.

In hopes of discouraging Iraqis from voting in Sunday's election — 21 months after Saddam Hussein's (search) downfall in April 2003 — insurgents have accelerated attacks, sending a message that if Iraqis suffer deaths and injuries on election day, "you have only yourselves to blame."

An American OH-58 Kiowa Warrior helicopter also crashed Friday night in southwestern Baghdad, U.S. officials said. There was no indication of hostile fire and no word on the fate of the crew, the officials said.

About 300,000 Iraqi, American and other multinational troops and police will provide security for the voting at 5,300 polling centers.

Voters will choose a 275-member National Assembly and governing councils in the 18 provinces. Voters in the Kurdish-ruled area will choose a new regional parliament.

Expatriate Iraqis began casting ballots amid tight security in early voting in 14 countries from Australia to Sweden to the United States.

There were few election posters or banners Friday but plenty of graffiti promising death to voters in Youssifiyah, a heavily Sunni Arab area south of Baghdad, where nostalgia for Saddam endures and hostility toward the United States is widespread.

Majority Shiites, who make up an estimated 60 percent of the population, are expected to turn out in large numbers Sunday, as are the Kurds. Iraqis will choose from among 111 lists of candidates for the National Assembly, rather than voting for individuals, and the ticket endorsed by the Shiite clerical hierarchy is expected to fare best.

Here and elsewhere in Sunni strongholds, however, insurgents do not have to do much to persuade people to boycott the election. Many Sunni Arabs, who make up about 20 percent of the population, believe Sunday's balloting will be tainted by the American occupation and Iranian meddling.

Many plan to stay home, threatening the legitimacy of the vote.

U.S. officials say security concerns — rather than political convictions — will largely determine who comes out to vote.

In Baghdad, U.S. Ambassador John Negroponte (search) insisted some Sunni Arabs will turn out to vote.

"Sunnis don't only live in some of these beleaguered provinces, they live here in Baghdad, they live in other parts of the country," Negroponte said on CBS' "The Early Show." "I think you're going to see participation across the board."

At the United Nations in New York, a spokesman for Secretary-General Kofi Annan said "everything has been set in place for a valid election process."

"We're in the middle of a process that will eventually, we hope, produce a democratic system of government, coming out of an autocratic system under Saddam Hussein," spokesman Fred Eckhard said.

A Western election adviser in Baghdad said Sunni turnout could be as high as 50 percent if election day violence is low and if the boycott call is not heeded. But it could also be as low as 15 percent, the adviser said on condition of anonymity.

"We applaud the courage of ordinary Iraqis for their refusal to surrender their future to these killers," President Bush said in Washington.

To discourage turnout, Sunni-led insurgents have stepped up attacks against polling centers, candidates and electoral workers across Iraq. In response, U.S. and Iraqi forces have accelerated sweeps to detain suspected insurgents. Residents say dozens of men have been rounded up in recent days.

To try to bolster public confidence, Iraqi officials Friday announced the arrests of three more purported lieutenants of Jordanian terrorist Abu Musab al-Zarqawi, including his military adviser and chief of operations in Baghdad.

The arrested al-Zarqawi associates included Salah Suleiman al-Loheibi, the head of his group's Baghdad operation, who met with al-Zarqawi more than 40 times over three months, said Qassim Dawoud, a top security adviser to Iraqi Prime Minister Ayad Allawi.

Dawoud said Ali Hamad Yassin al-Issawi, another associate, also was captured. Dawoud said the two arrests took place within the past several weeks.

Al-Zarqawi's military adviser, a 31-year-old Iraqi named Anad Mohammed Qais, 31, also was captured, said Deputy Prime Minister Barham Saleh.

"We are getting close to finishing off al-Zarqawi and we will get rid of him," Saleh said.

Despite Saleh's assurances, al-Zarqawi's group posted a new Web message Friday warning Iraqis they could get hit by shelling or other attacks if they approach polling stations, which it called "the centers of atheism and of vice."

"We have warned you, so don't blame us. You have only yourselves to blame," it said.

On Friday, a bomb went off near a ballot center in Iskandariyah, the latest sign of electoral violence in the town of 200,000 people south of Baghdad. U.S. soldiers also arrested a prominent Sunni Arab cleric and two of his brothers, raiding their home at dawn.

The cleric, Sheik Abbas Jassim, is a senior member of the Association of Muslim Scholars, an influential Sunni group that has called for an election boycott.

Small cracks, however, have begun to appear in the Sunni shunning of the vote.

In Diyala province, the Iraqi Islamic Party — the country's largest Sunni party — has partially reversed its decision to withdraw from the election, asking supporters to vote for local government candidates, local party leader Hussein al-Zobeidi said.

A Diyala tribal chief, Taha Aziz Hussein, said fear of election day attacks and anger at a wave of U.S. arrests undoubtedly will hurt turnout. But he added: "I am anticipating pockets of success in parts of Diyala."

In Kirkuk, Sunni Arab tribal leaders also urged followers to participate in the local government election, saying they wanted to deny Kurds domination of the oil-rich city.

"We cannot stay home and let the Kurds vote," said one tribal chief, Abdul-Rahman al-Monshid. "We shall participate, so it can never be said that this is a Kurdish city."

Key Shiite candidates repeatedly have sought to reassure Sunni Arabs that, regardless of how they fare in the vote, they will be included in the next government and the drafting of a new constitution.

But that holds little appeal for those who see the U.S.-sponsored political process as just an American scheme to install a loyal, Shiite-dominated government.

"The outcome of these elections has already been decided," Saleh Eid, a landowner from Mahmoudiya, told a group gathered at the house of tribal chief Adnan Fahd al-Ghiriri. "I believe 99 percent of us here will not vote."

Al-Ghiriri, a retired police office, agreed.

"I will stay home on Sunday," he said. "It's the safest place."