At least 16 near-simultaneous explosions struck cities and towns across Iraq, killing up to 43 people and wounding more than 200.
The blasts are the latest in a spate of violence ahead of next week's Arab League summit in Baghdad.
The summit is seen as the country's debut on the regional stage following the withdrawal of US troops in December.
The Iraqi government is anxious to show it can reinforce security to host its neighbours.
The worst attack occurred in the southern holy Shia city of Kerbala, where two explosions killed at least 13 people and wounded 48.
In the northern city of Kirkuk, a car bomb exploded near a police headquarters, killing seven and wounding 30.
A suicide car bomber killed three people and wounded 21 in central Baghdad.
Blasts also occurred in Baiji, Samarra, Tuz Khurmato, Daquq and Dhuluiya to the north, in Ramadi in the west, and Hilla, Latifiya and Mahmudiya in the south.
Police also defused bombs in Baquba and Falluja.
Although violence in Iraq has declined since the height of sectarian fighting in 2006 and 2007, bombings and shootings still occur on a daily basis nine years after the US-led invasion.
This was Iraq's deadliest day in nearly a month, and the breadth of coordinated bombs in more than a dozen cities showed an apparent determination by insurgents to prove that the government cannot keep the country safe ahead of the summit.
The Arab League summit is due to be held in Baghdad on 27-29 March.
