Strict security measures have been introduced to prevent attacks and minimise bloodshed during the main vote on Thursday, similar to those enforced during two earlier elections this year.
Airports and borders will shut from Wednesday until Friday or Saturday, curfews extended and a ban on carrying weapons imposed for even those with permits.
A five-day public holiday will also be in effect.
Monday's special polling booths opened at 7am local time and are due to close at 5pm.
In the northern Kurdish city of Sulaimaniyah, almost 20,000 hospital patients, detainees and security forces had registered to vote in 499 special polling booths, said Sheikh Latif, a local electoral commission official.
Overseas voting for Iraqi expatriates begins on Tuesday and lasts for three days in 15 participating countries.
Security clampdown
Iraqi authorities are hoping to keep insurgent violence at bay and protect voters just four days from a landmark parliamentary election where Iraqis will choose leaders for the next four years.
The strict security measures were unveiled as Britain admitted it was no closer to resolving the latest hostage crisis after a threatened deadline ran out to kill four Western aid workers.
With Iraq gripped by an insurgency targeting foreigners and Iraqis alike, Interior Minister Bayan Jabr Solagh said the aim of the security crackdown was to minimise bloodshed during Thursday's election.
"We are hoping for a calm day as during the referendum…." on a new constitution when the city of six million people took on the air of a ghost town, Mr Solagh said.
Iraq's 15.5 million voters are to elect their first full-term legislature since Saddam Hussein's regime fell in April 2003.
The Organisation of the Islamic Conference has urged the Sunni Arabs who boycotted January 30 legislative polls to participate "en masse and effectively" in the upcoming poll.
Iraqi Prime Minister Ibrahim Jaafari, in an interview with the BBC, expressed optimism that many Sunnis would participate.
The Sunni minority, which controls barely 20 seats in the transitional 275-member parliament, is desperate to win as large a mandate as possible this time in a government almost inevitably set to be dominated by Shi'ites.
Iraq's electoral commission has pressed for a halt to military operations, especially in predominantly Sunni areas, in order to calm the situation.
Mounting violence
But as the election campaign entered the final stretch, candidates issued bitter accusations over mounting political violence.
Killings and attacks targeting electoral candidates, their staff and offices have swept the country ahead of the vote.
Former prime minister Iyad Allawi, campaigning on a non-sectarian ticket, has accused the government of leading Iraq to the brink of civil war.
A key Sunni group, the Islamic Party, claimed authorities had massively violated campaign rules and were planning to manipulate election results.
In the past two weeks, two Islamic Party officials have been murdered, a Kurdish candidate killed and at least three Shi'ite officials found dead.
Western officials admit that intimidation and violence are a problem but say they are no worse than during polls in the Balkans or Algeria in the 1990s.
"Yes it's a problem absolutely but it's not really considered unexpected considering the context in which these elections are taking place," a diplomat told reporters.
Hostage silence
Silence surrounded the fate of four Western hostages after a deadline set by their captors ran out, leaving the British government to admit it had drawn a blank despite making round-the-clock efforts for its abducted national.
"We are doing ... everything possible to try and make sure his life is saved and that of his colleagues is protected," British Defence Secretary John Reid said.
"But we have no further indication of any movement as of this morning."
British grandfather Norman Kember, 74, US national Tom Fox, 54 and their Canadian colleagues from the Christian Peacemakers Team, James Loney, 41, and Harmeet Singh Sooden, 32, were abducted in Baghdad two weeks ago.
Extremist Islamists have threatened to kill the hostages unless prisoners held by US and Iraqi authorities are released.
Also missing are a German archeologist, French engineer and US security contractor.
Another Islamic group claimed on Thursday to have killed the security contractor, although there has been no confirmation of the death of the missing American, Ronald Schulz.