"Participation has been excellent," Qassim Abudi, the administrative director of the Iraq High Electoral Commission, said in Baghdad.
Nuri al-Maliki, the Iraqi prime minister, is seeking to increase his regional influence via the polls to counter rival Shia parties ahead of parliamentary elections later this year.
Sunni parties, many of whom boycotted the last provincial polls in 2005, are also aiming to gain a greater share of local power.
"Brothers and sisters, only hours are left separating us from this unforgettable day, election day," al-Maliki said at an election rally in the southern city of Amara, before voting started.
"What makes us happy is the preparations we are seeing today - a slap in the face of those who are betting that Iraqis will not go to the ballot boxes because they are despairing."
"Whereas the January 2005 elections helped put Iraq on the path to all-out civil war, these polls could represent another, far more peaceful turning point," Robert Malley, the Middle East and North Africa programme director at the International Crisis Group, said.
"Despite likely shortcomings, the elections may begin to redress some of the most severe problems associated with the 2005 vote, assuring fairer representation of all segments of the population," said Malley.
The initial round of voting was decided upon to prevent electoral fraud and ensure security and logistical rigidity.
With UN assistance, elections are to be held in 14 of Iraq's 18 provinces in total, with about 15 million citizens eligible to vote for 440 seats.
The country's borders will be closed the day before Saturday's final stage of voting and transport bans and night-time curfews will be in place.