An Iraqi stands in front of a building heavily damaged in a car bomb explosion in Baghdad, Iraq. Two car bombs exploded Sunday in an outdoor market in the predominantly Shiite area of Baghdad, killing at least 63 people and injuring scores. It was the deadliest attack since U.S. and Iraqi forces began a major security push last week across the capital.

Baghdad, Iraq ? Militants struck back Sunday in their first major blow against a U.S.-led security clampdown in Baghdad with car bombings that killed at least 63 people, left scores injured and sent a grim message to officials boasting that extremist factions were on the run.

The attacks in mostly Shiite areas – twin explosions in an open-air market that claimed 62 lives and a third blast that killed one – were a sobering reminder of the challenges confronting any effort to rattle the well-armed and well-hidden insurgents.

Instead, it was the Iraqi commanders of the security sweep feeling the sting.

Just a few hours before the blasts, Lt. Gen. Abboud Qanbar led reporters on a tour of the neighborhood near the marketplace that was attacked and promised to “chase the terrorists out of Baghdad.” On Saturday, the Iraqi spokesman for the plan, Brig. Gen. Qassim Moussawi, said violence had plummeted by 80 percent in the capital.

Prime Minister Nouri al-Maliki condemned the bombings as a desperate act by “terrorists” and “criminals” who sense they are being squeezed.

“These crimes confirm the defeat of these perpetrators and their failure in confronting our armed forces, which are determined to cleanse the dens of terrorism,” al-Maliki said in a statement.

U.S. military chiefs have been much more cautious. They have insisted the security drive, begun last week, may take months to make clear gains and that counterpunches from militants were likely every step of the way.

The ones dealt Sunday came from the militants’ favored weapon of the moment: parked cars rigged with explosives.

The first blast tore through a produce market in the mostly Shiite area of New Baghdad, toppling the wooden stalls and leaving pools of blood and vegetables trampled in the chaos. Minutes later, another car bomb exploded near a row of stores.

More than 129 people were injured, including many women who were shopping, said police and rescue officials who spoke on condition of anonymity because they are not authorized to speak to the media.

Victims were carried to hospitals on makeshift stretchers or in the arms of rescuers.

Another car bomb in the Shiite enclave of Sadr City left at least one dead and 10 wounded, police said.

It was by far the deadliest day since the security sweeps began. On Thursday, a string of car bombs killed seven civilians on the first full day of house-to-house searches for weapons and suspected militants.

The U.S.-led teams have faced limited direct defiance as they set up checkpoints and comb neighborhoods. But that could change as they move into more volatile sections of the city. The next could be Sadr City, a stronghold of the Mahdi Army militia loyal to radical Shiite cleric Muqtada al-Sadr.

U.S. soldiers pressed closer to Sadr City on Sunday and the reception changed noticeably. In previous days, Shiite families opened their doors to welcome the troops – feeling that the American presence would be a buffer against feared attacks from Sunni militia.

On Sunday, in areas closer to Sadr City, parents slapped away the candy and lollipops given to children by soldiers.