Angry survivors hurled stones at Iraqi soldiers at the site of one of the blasts in Sadr City after troops fired shots in the air to disperse crowds of people trying to care for the injured, witnesses said.

No group claimed responsibility. But a U.S. military spokesman said the attacks appeared to be a coordinated assault by al-Qaida, saying the nature of the targets was consistent with past attacks.

Twelve people were killed in a market in western Baghdad when two car bombs exploded almost simultaneously, while 32 others were wounded, an Iraqi police official said.

Burned hulks of cars and twisted metal were scattered across the marketplace, as Iraqi soldiers and police officers surrounded the bombing site, driving off onlookers and journalists.

The day's violence started with a car bomb at 7:30 a.m. in the center of the capital that killed at least six people and wounded 17, said a police official, who said most of the victims were day laborers seeking work.

Later, a bomb in a parked car exploded at a market in the Shiite slum of Sadr City, also killed 12 people, including three women and four children, and wounded 37 others, said Iraqi police and medical officials. Within minutes, another bomb went off at another eastern Baghdad market, killing three more people and wounding 15, said a security official.

A roadside bomb targeting a three-vehicle police convoy carrying an Interior Ministry official in eastern Baghdad killed three people, including two of the official's guards, and wounded 12 others, said another police official.

Anger against Iraqi security forces boiled up after the blast in Sadr City, scene of heavy fighting last year between U.S.-Iraqi forces and militiamen loyal to radical Shiite cleric Muqtada al-Sadr.

Adnan al-Sudani, 37, said he and others rushed to the scene of the blast as black smoke billowed from the bombing site.

"We saw several people dead, and some were burned. We began to lift them along with the wounded into civilian cars to take them to nearby hospitals," he said. "When Iraqi army forces arrived, they began firing randomly on people to disperse them. But angry people began to throw stones at them."

U.S. officials insist that violence has fallen by 90 percent since the high point in 2007, but a recent uptick in attacks has raised concern that extremists may be regrouping.

"The nature of the attacks and targets are consistent with past al-Qaida in Iraq attacks," said a U.S. spokesman, Maj. David Shoupe. "We see this as a coordinated attack by terrorists against predominantly Shia targets that they gauge as vulnerable to instigate sectarian violence."

The U.S. military has begun to remove troops from Baghdad before the June 30 deadline for leaving the cities, as required by the U.S.-Iraq security agreement that took effect this year.

Tension has been increasing in Baghdad in recent weeks between the Shiite-led government and mostly Sunni paramilitary groups that the U.S. organized to provide security in their neighborhoods.

Last month, Iraqi troops put down an uprising by a paramilitary group in central Baghdad that began after their leader was arrested. The paramilitaries, known as Awakening Councils or Sons of Iraq, also complain that a number of their members have been arrested in what they fear is an attempt to marginalize them.

Also Monday, the U.S. military announced that a U.S. soldier was killed in action the day before in Diyala province, where insurgents remain active.

It was the first combat death suffered by U.S. forces in Iraq since March 16, when a soldier was killed by a roadside bomb in Baghdad.

Also in Diyala province, unidentified gunmen killed two Kurds Sunday night in a drive-by shooting in Jalula, 80 miles (125 kilometers) northeast of Baghdad, Iraqi army Capt. Sarjo Ahmed said Monday.

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Associated Press Writer Chelsea J. Carter contributed to this report.