The attack on the pilgrims in the Dura area of south Baghdad took place at a tent where they are served food and drinks on their way to the shrine city of Karbala, and also wounded at least 21 people, officials said.
Hundreds of thousands of people make pilgrimages to Karbala, many of them on foot, during the 40 days after the annual commemoration marking the death of the Prophet Mohammed's grandson, known to Shiites as Imam Hussein. The 40th day, known as Arbaeen, falls on December 23 this year.
The throngs of pilgrims on the roads make for an easy target, and they have been hit by a series of attacks in recent days.
Yesterday, a suicide bomber targeted Shiite pilgrims in Khales, north of Baghdad, killing five people and wounding 10.
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The toll would likely have been higher were it not for the selfless actions of a policeman who embraced the bomber just before the attack, in an effort to shield others from the blast.
On Tuesday, two attacks against pilgrims in and near Baghdad killed at least eight people, and on Monday two car bombs targeting pilgrims south of the capital killed at least 24 people.
The Sahwa militias are made up of Sunni Arab tribesmen who joined forces with the United States from late 2006, helping to bring about a significant reduction in violence.
They are frequently targeted by Sunni militants, who consider them traitors.
Violence in Iraq has surged this year to levels not seen since 2008, when the country was just emerging from a brutal sectarian conflict.
More people were killed in the first eight days of this month than in all of December last year.
Analysts say that widespread discontent among Iraq's minority Sunni Arab community is a major factor fuelling the surge in unrest.
The civil war in neighbouring Syria, which has bolstered extremist groups, has also played a role.