Around 2,500 of the volunteers arrived in Ramadi, located 115 kilometers (70 miles) west of the capital, on Friday and are to be joined by the remaining 1,500 on Saturday, said Gen. Rasheed Flayeh, the commander of operations in Anbar province. The men are being ferried out to Ramadi from Baghdad by helicopter, he added.
The vast majority of volunteers are Shiites who have answered a call from the country's top Shiite cleric, Ayatollah Ali al-Sistani, to defend Iraq from the militants who have overrun of much of the country's north and west over the past month. The Sunni militant blitz is led by the Islamic State extremist group, which has unilaterally declared the establishment of an Islamic state ruled by Shariah law in the territory it controls straddling the Iraq-Syria border.
The government's reliance on Shiite militias - who have deployed in sizeable numbers to several cities across the country - to help counter the threat from Sunni militants has ramped up sectarian tensions, and helped fuel fears that Iraq could return to the wholesale sectarian bloodletting that engulfed the country in 2006 and 2007.
There are already worrying signs of such violence.
Human Rights Watch said Friday that Iraqi security forces and government-affiliated militias appear to have killed at least 255 prisoners in six cities and villages since June 9. It said five of the mass killings took place when security forces were fleeing as militants advanced, and that the vast the prisoners killed were Sunni.
Most members of the security forces and militias are Shiite. The six incidents appear to be aimed at avenging the deaths of Shiites captured and killed by the Islamic State group, Human Rights Watch said.
There is also evidence the militants have carried out mass killings. The Islamic State group posted graphic photos online last month showing the militants killing dozens of police and soldiers. The Iraqi military confirmed the photographs and said around 170 soldiers were killed. Human Rights Watch put the number between 160 and 190.
Ramadi is the capital of Anbar, an overwhelmingly Sunni province and one of the most active battle fronts in Iraq. The Islamic State group and other Sunni militant groups seized control of the Anbar city of Fallujah and parts of Ramadi in January. The government has since reasserted its control of Ramadi, but Fallujah remains in insurgent hands.
The militant onslaught over the past month has touched off Iraq's worst crisis since the last U.S. troops left in 2011 and sapped public - and international - confidence in Shiite Prime Minister Nouri al-Maliki.
The Iraqi leader's opponents, and even many of his former allies, accuse him of trying to monopolize power and alienating the Sunni community, and are pushing him to step aside and not seek a third consecutive term. Despite the pressure, al-Maliki has vowed not to withdraw his candidacy for the prime minister's post, and points to his State of Law bloc's capturing the most seats in April elections to claim he has a mandate.
Iraq's new parliament is scheduled on Sunday to hold its second session since the elections amid hopes that lawmakers can quickly decide on a new prime minister, president and speaker of parliament - the first steps toward forming a new government. It failed to make any progress in its first session, and postponed its second session until Sunday.
On Saturday, the U.N. special envoy to Iraq, Nickolay Mladenov, warned that failure to quickly elect new leadership "risks plunging the country into chaos."
"It will only serve the interests of those who seek to divide the people of Iraq and destroy their chances for peace and prosperity," he said. "Iraq needs a team that can bring people together. Now is not the time for mutual accusations, now is the time for moving forward and compromising in the interest of the Iraqi people."
He urged lawmakers to attend Sunday's session and succeed in choosing a new speaker of parliament.