Iraq's first free elections in decades moved forward Sunday amid a wave of attacks by insurgents bent on disrupting the vote.

Despite heavy security measures, attacks during the first few hours of voting left at least 20 people dead, most of them in a series of suicide bombings on polling stations in Baghdad.

One of the deadliest suicide attacks killed four people. A mortar attack near a polling center in Sadr City, home to most of the capital's Shi'ite Muslims, killed another four people.

Blasts also were reported in other cities, including Baquba and Basra.

Despite the violence, Iraqi election officials said they are confident that "the day will pass successfully." The United Nations' election adviser in Iraq, Carlos Valenzuela, says initial reports indicate that turnout is good, even exceeding expectations in some areas.

Prime Minister Iyad Allawi, who voted in Baghdad's heavily fortified Green Zone, says the election is the start of a new era in Iraq. President Ghazi al-Yawar, among the first to vote, urged all Iraqis to go to the polls.

Officials say more than 5,000 polling stations are open nationwide for the 14 million Iraqis who are eligible to vote.

Voting began sluggishly in some areas as people waited to see whether feared attacks would materialize. By mid-day, turnout increased and was brisk in some Shi'ite Muslim communities, both in Baghdad and in the south.

Televised images showed voters arriving at sites on foot and going through a strict security check before approaching the voting booths.

Long lines were reported in Baghdad's Sadr City, and in the Kurdish regions of northern Iraq.

Witnesses say there were no signs of voting in the Sunni Muslim strongholds of Fallujah, Ramadi and Samarra, west and north of Baghdad. In the mixed but heavily Sunni city of Mosul in northern Iraq, the streets were also deserted and polling centers empty.

Cities were sealed off and a curfew was imposed before the vote. Despite unprecedented security measures, violent attacks killed 17 Iraqis on Saturday. And a rocket attack killed two Americans at the U.S. Embassy in Baghdad's Green Zone.

Thousands of Iraqis living abroad have cast votes in 14 countries from Australia to Europe to the United States.