The US military is sending heavy troop deployments onto the streets
A full-scale security clampdown is being rolled out in Iraq ahead of regional elections this weekend.
It comes after gunmen killed three candidates in separate incidents across the country, after a relatively violence-free campaign.
Millions of Iraqis are expected to vote for provincial councils, in the first country-wide elections since 2005.
They are being seen as a test of Iraq's improving stability and a guide to the general election later this year.
The shooting of Sunni election contenders happened on Thursday in Baghdad and in the northern city of Mosul. In Diyala province, a candidate and two campaign workers were also killed.
The level of violence around Iraq is significantly lower than in past years. The security measures, however, include closing Iraq's international borders, ordering traffic bans across Baghdad and major cities, halting air traffic and night-time curfews.
Hundreds of women, including teachers and civic workers, have also been recruited to help search women voters after a rise in female suicide bombers last year, according to the Associated Press.
Iraqi and US military commanders have in recent days warned that al-Qaeda poses a threat to the elections.
The International Crisis Group, which studies conflict-torn nations around the world, has emphasised the importance of the poll.
In a report, it said: "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."
The election is also being seen as a quasi referendum on the leadership of Prime Minister Nouri Maliki.
Four years ago, Iraq's Sunni Arabs boycotted the legislative election, allowing Shia and Kurdish parties to take control of parliament, but Sunnis are now expected to take part in large numbers.
Saturday's elections are being held in 14 of the country's 18 provinces, with more than 14,000 candidates competing for just 440 seats.
It is being organised by the United Nations and Iraq's Independent High Election Commission, with 800 international observers expected to oversee the balloting.
Iraq's provincial councils are responsible for nominating the governors who lead the administration and oversee finance and reconstruction projects. Security forces remain under federal government control.