Iraqi security forces and allied tribesman used Hellfire rockets to quell attacks by al-Qaeda-linked gunmen in two of the country's main Sunni cities.
The militants overran police stations and swept through the streets in Ramadi and Fallujah amid mounting sectarian tensions between minority Sunnis and the Shiite-led government.
The two cities in the Sunni heartland of Anbar province were once strongholds for militants battling US troops.
The Hellfire rockets were sent recently by the US to help the government fight al-Qaeda's Iraq branch, known as the Islamic State in Iraq and the Levant.
The heaviest fighting was in Fallujah, 60 kilometres west of Baghdad, where security officials said their forces were meeting particularly heavy resistance from al-Qaeda fighters.
In Ramadi, security forces took back several police stations. There was no immediate word on casualties, but footage released by the military showed forces firing Hellfire missiles at militant positions.
The militants appeared to be trying to exploit Sunni anger after authorities arrested a senior Sunni politician accused of terrorism and dismantled a months-old sit-in by Sunnis in Ramadi protesting over discrimination by the government. Those moves added new fuel to sectarian violence that has escalated since the US withdrawal.
In a concession to Sunnis after the sit-in was dispersed, Prime Minister Nouri al-Maliki pulled troops from Anbar, allowing local police to take over security duties. That was a main demand of discontented Sunni politicians who see the army as a tool Maliki uses to target his rivals and consolidate power.
But soon after the pull-out, the militants launched the simultaneous assaults in Ramadi, Fallujah and at least two other nearby towns. They seized police stations and military posts, freed prisoners and fanned out in the streets, setting up checkpoints.
Some were seen cruising in captured security forces' vehicles, waving al-Qaeda banners.
Maliki ordered military reinforcements back in and called on Sunni tribesmen to help the fight against the militants.
In another apparent move to maintain Sunni support, security forces arrested a controversial Shiite cleric who leads an Iranian-backed militia.
Sunnis have long accused the government of targeting only Sunni militant groups while blessing Shiite ones.
Interior Ministry spokesman Saad Maan Ibrahim said the cleric, Wathiq al-Batat, was arrested in Baghdad on Wednesday. Al-Batat has been wanted by the government since last year.
He took responsibility in November for firing six mortar shells at a region of Saudi Arabia bordering Iraq and Kuwait, describing it as retaliation for Saudi religious decrees that allegedly insulted Shiites and encouraged killing them. He also claimed responsibility for attacks on a camp hosting an Iranian opposition group. Al-Batat was previously a leader in Iraq's Hezbollah Brigades, not related to the better-known Lebanese Hezbollah.
Hezbollah in Iraq is believed to be funded and trained by Iran's elite Revolutionary Guard and was among the Shiite militias that targeted US military bases months before their withdrawal in December 2011.
Meanwhile, in new violence outside Anbar, a pickup truck laden with explosives blew up on a busy commercial street on Thursday evening in the city of Balad Ruz, 70 kilometres northeast of Baghdad, destroying several shops. At least 19 were killed and 37 wounded.
Also, a bomb stuck onto a public minibus exploded in Baghdad's Shaab district, killing four people and wounding six.
And three soldiers died when patrols were bombed in the northern city of Mosul.
Iraq has seen an increased wave of sectarian violence since last April, when security forces broke up a Sunni protest in a bloody crackdown.
Since then, militant attacks have swelled and Shiite militias have grown more active.
The United Nations said on Wednesday that last year saw Iraq's highest annual death toll since the worst of the sectarian bloodletting began to subside in 2007, with 7,818 civilians killed.
The UN's figures for both civilian and security force deaths over the year totalled 8,868.