Several loud explosions rocked central Baghdad, Basra, Mosul and Baquba shortly after the start of voting on Sunday.
The blasts came from different districts in the centre of the capital and police said an explosion had jolted the Mansur district, an upper class area of western Baghdad.
Iraqi police said the Mansur attack was a car bomb at a makeshift polling station in the Zahra school. Sources said six people were killed, including an Iraqi security member. Thirteen others were injured.
In northern Iraq, six explosions rocked Mosul early on Sunday but there were no reported casualties. One polling station visited by the media was empty.
Similarly, a mortar shell landed near a polling station in the southern city of Basra, but there were no reports of causalities.
In the northern city of Balad, a mortar attack on a polling station killed one woman and wounded another woman and her child.
Stolen police vehicles
Rafi al-Bayati, an independent Iraqi journalist in Baghdad, said about 20 explosions were heard throughout the city early on Sunday.
He said rockets appeared to have been launched from the al-Ghazaliya distict west of Baghdad directly hitting polling centres.
Iraqi police: The whereabouts of
the stolen vehicles are unknown
Al-Bayati also learned from local police sources that about 20 police vehicles and ambulances had been stolen in the past few days and believed to be rigged with explosives.
US military authorities had warned before the election that attacks and bombers were likely on election day.
Also in Baghdad, US forces opened fire on Iraqi security officers belonging to the interim interior ministry. Two were severely injured and taken to a nearby hospital, al-Bayati said.
Sadr City casualties
Meanwhile, mortar shells struck a polling station in Sadr City killing four voters and wounding seven others.
Earlier, about 10 women and 18 men lined up outside polling stations as similar scenes played out across Iraq. In the Shia shrine city of Najaf, five men were seen queuing outside one polling centre.
A few mortars exploded near the US military base in the northern city of Kirkuk shortly before the voting centres opened.
No Samarra vote
Meanwhile, the head of the local council in Samarra said no citizens would vote because of the poor security situation.
"Nobody will vote in Samarra because of the security situation," said Taha Husain, the head of Samarra's local governing council.
No employees turned up at polling centres in Samarra and police were not to be seen on the streets, an agency correspondent reported.
Police stood guard on Sunday and poll workers checked identifications at schools and other buildings serving as polling stations which opened to voters at 7am.
Baghdad's streets were deserted at dawn. The only activity on one stretch of empty road was an American Humvee racing across a street in response to a burst of gunfire
Ghazi al-Yawir was the first Iraqi
official to cast his ballot
Voters were able to cast their ballots at the estimated 5500 polling centres around the country, said Farid Ayar, the head of Iraq's Independent Electoral Commission (IEI).
Ziyad al-Samarrai, an Iraqi journalist, speaking to Aljazeera from Baghdad said voters headed towards few centres in Baghdad.
He added that voters had not yet approached polling centres in al-Yarmuk, al-Bayya, al-Shaab, al-Huriya districts and al-Rabia street.
Al-Samarrai went on to say that the heavy security presence caused fear among citizens more than the attacks themselves.
Iraq's interim President Ghazi al-Yawir became the first Iraqi official to vote.
Al-Yawir, draped in white and yellow robes, dropped his vote into a clear plastic ballot box at 7:05am (0405 GMT).
"Thank God, Thank God. Blessed are the Iraqi elections," he said. "We greet all Iraqi people and urge them not to give up their rights, to vote for Iraq, elect Iraq and not to give up on Iraq." Al-Yawir voted inside Baghdad's sealed off Green Zone.