At least 43 people were killed in car bomb explosions targeting Shi'ite Muslims in the Iraqi capital and the southern oil hub of Basra on Monday, police and medics said.
About 150 people have been killed in sectarian violence over the past week and tensions between Shi'ites, who now lead Iraq, and minority Sunni Muslims have reached their highest level since US troops pulled out in December 2011.
No group immediately claimed responsibility for the attacks. Iraq is home to a number of Sunni Islamist insurgent groups, including the al Qaeda-affiliated Islamic State of Iraq, which has previously targeted Shi'ites in a bid to provoke a wider sectarian confrontation.
Nine people were killed in one of two car bomb explosions in Basra, a predominantly Shi'ite city 20km (260 miles) southeast of Baghdad, police and medics said.
"I was on duty when a powerful blast shook the ground," said a police officer near the site of that attack in the Hayaniya neighbourhood.
"The blast hit a group of day labourers gathering near a sandwich kiosk," he told Reuters, describing corpses littering the ground.
"One of the dead bodies was still grabbing a blood-soaked sandwich in his hand."
Five other people were killed in a second blast inside a bus terminal in Saad Square, also in Basra, police and medics said.
In Baghdad, a parked car exploded in a busy market in the mainly Shi'ite eastern district of Kamaliya, killing seven people, police said.
A further 22 people were killed in blasts in Ilaam, Diyala Bridge, al-Shurta, Shula and Sadr City - all areas with a high concentration of Shi'ites.
Iraq's delicate intercommunal fabric has come under increasing strain from the conflict in neighbouring Syria, which has drawn Sunni and Shi'ite Muslims from across the region into a proxy war.
Syrian President Bashar al-Assad's main regional ally is Shi'ite Iran, while the rebels fighting to overthrow him are supported by Sunni Gulf powers Saudi Arabia and Qatar.
Iraq says it takes no sides in the conflict, but leaders in Tehran and Baghdad fear Assad's demise would make way for a hostile Sunni Islamist government in Syria, weakening Shi'ite influence in the Middle East.
The prospect of a possible shift in the sectarian balance of power has emboldened Iraq's Sunni minority, embittered by Shi'ite dominance since the overthrow of Saddam Hussein by US-led forces in 2003.
Thousands of Sunnis began staging street protests last December against Shi'ite Prime Minister Nuri al-Maliki, whom they accuse of marginalising their sect.
A deadly raid by the Iraqi army on a protest camp in the town of Hawija last month ignited a bout of violence that left more than 700 people dead in April, according to a UN count, the highest monthly toll in almost five years.
At the height of sectarian violence in 2006-07, the monthly death toll sometimes topped 3,000.