Iraq faces calls to end a political stand-off after a wave of attacks against Shiite Muslims killed at least 68 people, the worst toll in nearly five months.
The violence, which wounded more than 100, comes two weeks after a crisis erupted when the Shiite-led authorities charged Sunni Vice President Tareq al-Hashemi with running a hit squad soon after US troops pulled out.
The US and the European Union swiftly condemned the attacks.
US State Department spokeswoman Victoria Nuland told reporters in Washington that: "We consider them acts of terror."
"They are desperate attempts by the same kind of folk who have been active in Iraq trying to turn back the clock," she added in reference to sectarian strife in 2006 and 2007.
The EU top diplomat Catherine Ashton deplored the "death and destruction caused by these acts of terrorism, which can only exacerbate an already fragile political situation."
Ashton, who is the EU's high representative for foreign affairs, called on all Iraqi political groups to engage in an "inclusive and genuine dialogue ... in order to address their differences".
Thursday's attacks, which targeted only Shiites, were also quickly condemned by Iraq's parliament speaker Osama al-Nujaifi, a Sunni, as well as by Iraq's Shiite-majority neighbour Iran and UN special envoy Martin Kobler.
Nujaifi and Prime Minister Nuri al-Maliki held talks in Baghdad in the aftermath of the attacks, amid calls for a meeting of political leaders to resolve the festering stand-off.
The worst incident saw at least 45 people killed by a suicide attacker on the outskirts of the southern city of Nasiriyah as pilgrims were walking to the shrine city of Karbala for Arbaeen commemorations.
"Hospitals in Nasiriyah have received 45 killed and 68 wounded," said Hadi Badr al-Riyahi, head of the provincial health department in Dhi Qar, of which Nasiriyah is the capital.
Arbaeen marks 40 days after the Ashura anniversary commemorating the killing of Imam Hussein, one of Shiite Islam's most revered figures, by the armies of the Caliph Yazid in 680 AD.
Five bombings also struck two Shiite neighbourhoods of northern Baghdad, killing 23 people and wounding dozens.
In Kadhimiyah, two car bombs exploded at around 9:00 am (0600 GMT) at adjoining intersections, said interior and defence ministry officials.
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The blasts killed 14 people and wounded 37 others, the defence official said, while the interior ministry source put the toll at 15 dead and 31 wounded.
Several nearby vehicles, shops and the facade of a newly built hotel were badly damaged, an AFP journalist said.
"Where are the security forces?" shouted 60-year-old Ashur Abdullah at Al-Zahra intersection in Kadhimiyah.
"Where are the checkpoints? How did this happen here? The responsibility lies with the security forces."
In Sadr City, a booby-trapped motorcycle exploded at around 7:00 am near a group of day labourers waiting to pick up work, killing seven and wounding 20, the interior ministry official said.
A short time later, twin roadside bombs detonated near the district's main hospital as victims were being ferried in, killing two more people and wounding 15, the official said.
The defence ministry official confirmed the toll.
Security forces cordoned off the scenes of the blasts, and largely refused to allow journalists to enter, sparking the ire of residents.
"Why are you preventing the press and photographers from entering the scene?" shouted one man in Kadhimiyah who declined to be identified.
"Are you afraid that the world will see your failure?"
Thursday's death toll was the highest in Iraq since a spate of attacks on August 15, most of which were claimed by Al-Qaeda, left 74 people dead.
The violence was the second major strike against Iraqis since a political crisis erupted last month. On December 22, 67 people were killed across the country.
Maliki has backed off from threats to fire ministers from the Sunni-backed Iraqiya bloc who have boycotted the cabinet, the latest move in an apparent toning down of the crisis.
The stand-off was sparked by a decision to issue an arrest warrant for Hashemi on terror charges.
Hashemi, who is holed up in the northern autonomous Kurdish region, denies the charges, and his Iraqiya party has boycotted the cabinet and stayed away when parliament reopened on Tuesday.
Sunni Deputy Prime Minister Saleh al-Mutlak, also a member of Iraqiya, has decried Maliki as a dictator "worse than Saddam Hussein," and the premier has called for him to be sacked.
UN envoy Kobler, in a statement on Wednesday, "expressed concern about the current political stalemate in the country," and US Vice President Joe Biden has urged dialogue among top Iraqi leaders.
On December 18, US forces completed their withdrawal from Iraq, where there were once nearly 170,000 American troops on 505 bases.