A suicide bomber has killed at least 43 people and wounded another 40 in western Baghdad, Iraqi police told Al Jazeera.
The attack on Sunday occurred when members of the government-backed Sahwa militia lined up to be paid at an office in the Sunni district of Radwaniya.
In a seperate incident another suicide bomber detonated an explosive belt near a Sahwa office in Anbar province on the Iraqi-Syrian border, killing three and injuring six.
The Sahwa militia, or 'Awakening movements' took up arms against al-Qaeda in late 2006 with US backing.
Its fighters, recruited from among tribesmen and former anti-government fighters, are credited with turning the tide in the war against al-Qaeda in Iraq.
Control of the Sahwa passed to Iraq in October 2008, and their wages - said to have been cut from $300 under US leadership to $100 - have been paid, often late, by the Shia-led government.
In the past six months many Sahwa fighters and members of their families have been killed in revenge attacks.
"They have been often targeted in recent months for their role in combating al-Qaeda and other Sunni fighters," Al Jazeera's Rawya Rageh, reporting from Baghdad, said.
The attacks have not been claimed yet, but "all fingers are going to be pointed at al-Qaeda in Iraq at this point," Rageh said.
Four months after an inconclusive parliamentary election in March, Iraq has yet to form a new government as politicians bicker over who will lead it.
Analysts say the political vacuum is fuelling instability and various armed groups are exploiting it to their advantage.