Three female suicide bombers killed at least 28 people and wounded 92 in Baghdad as Shi'ite pilgrims flooded into the Iraqi capital for a major religious event, police said.
In the northern oil city of Kirkuk a suicide bomber killed at least 22 people and wounded 112 others at a demonstration against Iraq's provincial elections law, the US military said.
The attacks underscored the fragility of recent security gains in Iraq, where violence has fallen to four-year lows.
There was no immediate claim of responsibility for the Baghdad blasts, but al Qaeda has often targeted Shi'ite pilgrims taking part in religious events in Iraq. It considers Shi'ism -- the majority Muslim denomination in Iraq -- heretical.
"These blasts that happened today will increase our determination to finalize the ceremony of this visit and defeat terrorism," said pilgrim Taher Abd-Noor.
Prime Minister Nuri al-Maliki has increasingly urged foreign dignitaries and companies to come to Iraq in recent months, citing a rapidly growing army that has been able to keep the peace with minimal US backup.
The apparently coordinated explosions in Baghdad shattered months of relative calm in the city, and took place despite a heavy security clampdown ahead of the annual Shi'ite pilgrimage to the Kadhamiya shrine in the city's northwest.
At least one million people are expected to take part in the pilgrimage, which peaks tomorrow and marks the death of one of Shi'ite Islam's 12 imams. Security officials said improved security would boost visitor numbers compared to previous years.
The US military said it was possible that three suicide bombers had carried out the attacks, but did not specify whether they were women.
Al Qaeda has increasingly used women to carry out suicide attacks because they can often evade the more stringent security checks applied to men. Women have carried out more than 20 suicide attacks in Iraq this year.
Reuters television showed police, firemen and other workers washing blood and clearing debris from the street at the scene of one of blasts. A Reuters witness saw workers collecting pieces of flesh and body parts.
The blasts occurred near the Karrada district in central Baghdad, an area many pilgrims would pass through on their way to the shrine. Gunmen killed seven pilgrims in southern Baghdad on Sunday as they made their way to the shrine on foot.
Kirkuk stampede
In Kirkuk, Kurdish television footage showed thousands of people demonstrating against Iraq's provincial elections when an explosion prompted a rush for cover.
A Reuters witness said there was a stampede as police started to shoot into the air.
Tensions have been in high in the disputed oil-rich city ahead of provincial elections scheduled to take place this year.
Mosques called for people to donate blood, the Reuters witness said, adding that he had seen two badly burned bodies.
Kurds in the ethnically mixed city say it should belong to the largely autonomous Kurdistan region, but Arabs want it to stay under central government authority.
Last week, Iraqi President Jalal Talabani rejected a provincial election law as unconstitutional after Iraq's Kurdish lawmakers boycotted the parliament session that ratified it.
The law would have postponed voting in Kirkuk and it included an article carving out fixed seat allocations to each ethnic or sectarian group.
Sunni-Islamist al Qaeda has exploited ethnic faultlines in Iraq's north, where it has regrouped after being forced from its former strongholds in Baghdad and Iraq's West.
Although on the back foot, the US military has said it expects insurgents to attempt high-profile attacks to grab media attention and show they are still a potent force.