Al-Qaida linked militants have taken control of large swathes of two Iraqi cities from government forces, security officials said.
Gunmen with al-Qaida's Iraqi offshoot, the Islamic State of Iraq and Levant (known as Isis or Isil), have ousted security forces from half of Fallujah, 60km west of Baghdad, and have also taken over parts of the nearby city of Ramadi.
Tension mounted after authorities broke up a Sunni anti-government protest camp in Ramadi and arrested a Sunni lawmaker on terrorism charges.
Shiite prime minister Nouri al-Maliki ordered the military out of Ramadi's Anbar province and left security in the hands of local police to calm tensions.
However Islamist militants swiftly launched a wave of retaliatory attacks across the province, raiding police stations and military posts in at least four cities and towns.
A number of prisoners were freed and videos uploaded on YouTube showed gunmen setting fire to security force vehicles and driving through the streets waving al-Qaida banners.
Al-Maliki sent the military back in and urged local Sunni tribesmen who oppose al-Qaida to help put down the militants.
A bomb went off in a commercial street of Balad Ruz, a city 70km northeast of the capital, killing at least 16 people.
Another four people died in a separate attack in the northern city of Mosul, where an explosive device stuck on a public minibus exploded wounding at least six more.
A cluster of Sunni militant groups have increased attacks against the Shiite-led government, which they said discriminates against them.
Isis is believed to be responsible for a spree of deadly bombings that has killed thousands and pushed Iraq to the brink of civil war.
In what has been seen as an attempt to appease Sunni sentiment, security forces arrested a controversial Shiite cleric, Wathiq al-Batat, who leads an Iranian-backed militia.
According to the UN, almost 8,000 civilians and 1,050 members of the security forces were killed in violent attacks in 2013.