A wave of bomb attacks and shootings in Baghdad and north of the capital has killed at least 107 people, say security and medical officials.
Many of those killed were security forces - who appear to have been a prime target, correspondents say.
One of the worst-hit places was Taji, a Sunni neighbourhood some 20km (12 miles) north of Baghdad, where at least 41 people were killed.
At least 216 people were wounded on one of the bloodiest days of the year.
Some 19 Iraqi towns and cities were hit in the spate of apparently co-ordinated attacks.
Fatal bombings hit Shia districts in Baghdad. In the deadliest attack, a car bombing at a government building in Sadr city killed at least 16.
At least five car bombs hit the northern oil city of Kirkuk, and further north in the city of Mosul at least nine people died, reportedly including six soldiers.
Bombs and shootings in the restive province of Diyala killed 11.
Saadiya, Khan Beni-Saad, Tuz Khurmatu, Dibis, Samarra and Dujail were also said to have suffered attacks.
The attacks come days after a man purporting to be Abu Bakr al-Baghdadi, the leader of al-Qaeda in Iraq, declared a new offensive to retake areas the group retreated from before US soldiers left the country last December.
"The majority of Sunnis in Iraq support al-Qaeda and are waiting for its return," the man said, according to Associated Press, in an audio message posted on militant websites.
Analysts suggest al-Qaeda is seeking to deepen the sectarian political crisis in Iraq that is setting Sunni Arab and Kurdish leaders against their Shia counterparts.
They say Monday's violence seemed to aim at security forces and government offices - favourite targets of al-Qaeda.
The security forces suffered badly in Monday's attacks, with 15 soldiers reportedly killed in a single brazen attack on a base in Dhuluiya in Salaheddin province.
Police checkpoints were hit by car bombs, army bases were struck by mortar fire, and one policeman was even attacked in his home, says the BBC's Rami Ruhayem in Baghdad.
In Taji, a string of five or six explosions went off. When police arrived on the scene to help, another explosion struck - according to one report, a suicide bomber.
In all, at least 41 people were killed, including at least 14 police. A row of houses was completely destroyed and residents were trying to find victims in the rubble, said a reporter for AFP news agency.
Resident Ali Hussein lamented the number of ordinary people killed in Taji.
"What is the guilt of these poor people?" he asked.
"They are working to earn a living. It is a poor market and people were here to shop in this market when the blast happened. Why did this happen?"
There was no immediate statement from the Iraqi government.
On Sunday, bombings south of the capital killed at least 17.
Monday's swiftly rising death toll made it the deadliest day of the year.
Violence dipped in Iraq following the insurgency in 2006 and 2007, but sectarian violence has returned across the country in recent months amid the worsening political tensions.
At least 237 people were killed during June, making it one of the bloodiest months since US troops withdrew in December.