BAGHDAD, Iraq (AP) - Two suicide car bombs exploded in downtown Baghdad Sunday, killing at least 136 people and delivering a powerful blow to the heart of the fragile city's government in the worst attack of the year, officials said.

While violence has dropped dramatically in the country since the height of the sectarian tensions, such bombings like Sunday's demonstrate the precarious nature of the security gains and the insurgency's abilities to still pull off devastating attacks in the center of what is supposed to be one of Baghdad's most secure areas.

Black smoke could be seen billowing from the frantic scene, as emergency service vehicles sped to the area. Even civilian cars were being commandeered to transport the wounded to hospitals.

"The walls collapsed and we had to run out," said Yasmeen Afdhal, 24, an employee of the Baghdad provincial administration, which was targeted by one of the car bombs. "There are many wounded, and I saw them being taken away. They were pulling victims out of the rubble, and rushing them to ambulances."

The car bombs, which targeted the Justice Ministry and the Baghdad provincial administration, come as Iraq is preparing for elections scheduled this January, and many Iraqi officials have warned that violence by insurgents intent destabilizing the country could rise.

There have been no claims of responsibility so far, but massive car bombs have been the hallmark of the Sunni insurgents seeking to overthrow the country's Shiite-dominated government.

At least 25 staff members of the Baghdad Provincial Council, which runs the city, were killed in the bombing, said council member Mohammed al-Rubaiey.

The area where the blasts occurred is just a few hundred yards from the Green Zone that houses the U.S. Embassy as well as the prime minister's offices. The street where the blasts occurred had just been reopened to vehicle traffic a few months ago, in what was supposed to be a sign that safety was returning to the once devastated city.

The devastating attacks occurred just hours before Iraq's top leadership was scheduled to meet with heads of political parties on Sunday and reach a compromise on the disputed election law ahead of a crucial parliamentary vote in January.

The explosive-laden vehicles were sitting in parking garages next to the two government building, police said.

"They are targeting the government and the political process in the country," Maj. Gen. Qassim al-Mousawi, spokesman for the city's operations command center, told The Associated Press. He said the blasts were the work of suicide bombers who drove the vehicles into the parking lots, before blowing them up.

The blasts, which surpassed coordinated attacks against two government ministries in August that killed more than 100 people, appeared to be a blow to Prime Minister Nouri al-Maliki who has staked his reputation and re-election hopes on returning security to the country.

Al-Maliki toured the blast sites later in the day.

Sunday's explosions also injured nearly 600 people who were taken to six area hospitals. Medical officials, speaking on condition of anonymity because they were not authorized to speak to the media, gave the death toll.

Video images captured on a cell phone showed the second blast going off in a massive ball of flames, followed by a burst of machine gun fire.

"This is a political struggle, the price of which we are paying," said provincial council member al-Rubaiey. "Every politician is responsible and even the government is responsible, as well as security leaders."

Three American security contractors, working for the U.S. embassy in Baghdad were injured in the blasts, but no American embassy personnel were killed, said Philip Frayne, an embassy spokesman. Frayne could not immediately provide details about who the contractors were escorting to the site, which company they worked for or, or the nature of their injuries.

The explosions were just a few hundred yards from Iraq's Foreign Ministry which is still rebuilding after massive bombings there in August. The bombings were a devastating blow for a country that has seen a dramatic drop in violence since the height of the sectarian fighting in 2006 and 2007.

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Associated Press Writer Rebecca Santana contributed to this story.