Candidates in Iraq's upcoming election were not allowed to campaign Wednesday in order to give Iraqis a day to reflect before they go to the polls to choose who will lead their country for the next four years.

Tightened security measures were in effect throughout the country, including a traffic ban in Baghdad that created an eerie quiet in parts of the city. The country's borders and airports were closed and an extended nighttime curfew was in effect.

Despite the security measures, there were a few reports of attacks across the country. In Mosul, a roadside bomb exploded as a police patrol passed, killing two officers and wounding four others, a hospital official said. And near Fallujah, police said gunmen shot up four polling stations, but there were no reports of casualties.

In Ramadi, Marines conducting security sweeps detected a large improvised explosive device, or IED, which had been placed at an intersection. A FOX News crew shot video of the Marines disarming the device and later blowing it up in a controlled explosion.

But as U.S. and Iraqi troops delivered ballots and prepared polling stations, the relative calm was broken by angry Shiites who took to the streets to protest a televised slur involving the country's religious leadership.

Up to 15 million Iraqis were to choose 275 members of the new parliament from among 7,655 candidates running on 996 tickets, representing Shiite, Sunni, Kurdish, Turkomen and sectarian interests across a wide political spectrum.

Polls open at 7 a.m. Thursday (11 p.m. EST Wednesday) and close at 5 p.m. (10 a.m. EST). Some preliminary returns were expected late Thursday, but final, complete returns could take days if not weeks.

"Let us make tomorrow a national celebration, a day of national unity and victory over terrorism and those who oppose our democratic march," President Jalal Talabani told a nationwide television audience.

Election of the new parliament, which will serve a full four-year term, marks the final step in the U.S. blueprint for democracy, climaxing a process that included the transfer of sovereignty last year, selection of an interim parliament Jan. 30 and ratification of the new constitution in October. The new parliament well name a government, including a new prime minister.

For the Bush administration, the stakes were nearly as high as for the Iraqis. A successful election would represent a much needed political victory at a time of growing doubts about the war within the U.S. public.

"We are in Iraq today because our goal has always been more than the removal of brutal dictator," President George W. Bush told the Woodrow Wilson International Center for Scholars. "It is to leave a free and democratic Iraq in his place."

A coalition of Shiite religious parties, which dominate the current government, is expected to win the biggest number of parliament seats.

The Bush administration hopes that more Sunni Arabs, who form the backbone of the insurgency, will win seats and help establish a government that can lure Sunnis away from the insurgency.

That would make it possible for the United States and its international partners to begin to draw down their troops next year.

In the Jan. 30 elections, many Sunnis boycotted the election, enabling the rival Shiites and Kurds to dominate the current legislature - a move that sharpened communal tensions and fueled the Sunni-led insurgency.

This time, however, more Sunnis Arabs are in the race, and changes in the election law to allocate the majority of seats by district all but guarantee strong Sunni representation.

More than 1,000 Sunni clerics called on their followers to vote, and major insurgent groups, including Al Qaeda in Iraq and the Islamic Army in Iraq, have pledged not attack polling stations, even though they oppose the political process. Insurgent threats kept many Sunnis at home in January, even though overall nationwide turnout was nearly 60 percent.

Nevertheless, tens of thousands of Iraqi soldiers and police will guard polling stations, with U.S. and other coalition forces standing ready to intervene in case of trouble. U.S. troops and sniffer dogs checked thousands of polling stations this week before sealing them and handing over control to Iraqi police.

As a sign of Sunni interest, mosques, walls, houses and lamp posts in Baghdad's Sunni district of Azamiyah were festooned with posters of Sunni candidates. In January, few people in Azamiyah voted, and some polling stations didn't even open.

The International Republic Institute, a pro-democracy group linked to the U.S. Republican party, predicted Wednesday that about 93 percent of eligible voters would show up at the polls.

Still, U.S. officials warned that a successful election alone will not end the insurgency. That will take a government capable of reconciling Iraq's disparate groups. U.S. officials are anxious to avoid protracted negotiations to choose a new prime minister and Cabinet - a process that dragged for three months after the January ballot.

"I think the elections are a positive step but it not be enough to ensure stability, more steps need to be taken. There should be a good government that represents all Iraqis and the security forces also should be formed by all Iraqi sects," U.S. Ambassador Zalmay Khalilzad told Al-Jazeera television.

Khalizad's comments about the security forces referred to longstanding Sunni Arab complaints that the Shiite-dominated army and police have abused Sunnis. On Tuesday, Khalilzad told reporters that at least 120 abused prisoners had been found in two detention centers run by the ministry since November.

On the eve of the election, sectarian tensions swelled over what Shiite political parties considered an offensive remark made by an Iraqi Shiite panelist during a talk show on Al-Jazeera television. Panelist Fadel al-Rubaei said Shiite clerics should not take part in politics, and he accused them of conspiring with the Americans against the mostly Sunni insurgents.

The statements angered many Shiites, including many who did not see the Al-Jazeera broadcast but saw reports of it on an Iraqi station, Al-Furat, owned by the biggest Iraqi Shiite party which used the report to fire up supporters on the election eve.

Abdul-Aziz al-Hakim, head of the Supreme Council for the Islamic Revolution in Iraq, condemned violations against Shiite religious leaders "broadcast by one of satellite channels known for its hatred to the Iraqi people."

Hours later, thousands of people chanted anti-Al-Jazeera slogans in the streets of the Baghdad neighborhoods of Sadr City and Karradah, and in major cities throughout the Shiite south.

In Nasiriyah, Shiite protesters set fire to a building housing the offices of former interim Prime Minister Ayad Allawi, a secular Shiite, and the Iraqi Communist Party.

"The headquarters was attacked by militiamen who broke inside and set fire to the building. This is a terrorist act that contradicts democracy and this is the reason we are calling for eliminating the militia groups in Iraq," Allawi spokesman Thaer al-Naqib told The Associated Press.

Officials at the Qatar-based Al-Jazeera were not available for comment. But Baghdad correspondent Atwar Bahjat told The Associated Press that she resigned from her job "in protest of what the guest of the station said."

Meanwhile, rumors swept Baghdad that a tanker truck filled with thousands of blank ballots had been smuggled into the country from Shiite-dominated Iran. Many Sunnis consider Shiite political parties as agents of Iran.

The Interior Ministry denied any attempt to smuggle ballots into the country, and the election commission said the only trucks in the area were its own delivering electional materials to polling stations. Election official Farid Ayar suspected the rumor was intentionally floated by undisclosed political groups.

Nevertheless, the report spread widely throughout Baghdad and other cities. Zaid Baqir, a 26-year-old Shiite bank employee in Baghdad, said the rumor had discouraged him from voting.

"I was so eager preparing myself to support the political process and defeat terrorism, but now I am not," Baqir said. "I don't trust this government and the whole atmosphere surrounding this election."

His wife Duha said she didn't believe the denials.

"They already tried hard to deny the existence of any secret prisons to torture Iraqis so it's typical of them to deny the existence of the tanker truck," she said.

The Associated Press contributed to this report.