BAGHDAD, Iraq - A huge car bomb exploded Saturday at a bustling outdoor market in a Shiite district of Baghdad, killing at least 66 people and injuring about 100 in the deadliest attack since the new national unity government took office six weeks ago.

Shortly before the midmorning blast, gunmen seized a female Sunni legislator at a checkpoint in a Shiite area of the capital. Both attacks represented a major challenge to Prime Minister Nouri al-Maliki's vow to curb sectarian violence that threatens to plunge Iraq into civil war.

In a audiotape posted Saturday on the Internet, al-Qaida leader Osama bin Laden urged Iraqi militants to continue the fight “and don't leave Baghdad, otherwise all the capitals in the region will fall to the crusaders,” meaning the Americans.

The market bombing appeared timed to cause a maximum number of casualties among civilians doing their weekend shopping. It blew some victims onto the roofs of nearby two-story buildings and sent a plume of gray smoke billowing into the sky above Sadr City, stronghold of radical cleric Muqtada al-Sadr.

Rescuers sprinted through streets littered with debris of shattered market stalls. Emergency crews bundled the wounded into ambulances and rushed them to hospitals. Late Saturday, Iraqi state television reported that 66 people died and about 100 were wounded.

No group claimed responsibility for the bombing, the deadliest since an April 7 suicide attack that killed 87 people at a Shiite mosque in Baghdad. But bystanders blamed Sunni extremists, such as al-Qaida in Iraq, whose leader Abu Musab al-Zarqawi was killed in a U.S. airstrike June 7.

“Give us the green light to attack and finish them for once and for all,” a group of young men shouted, referring to Sunni militants.

A few miles from the blast, gunmen seized lawmaker Tayseer al-Mashhadani and seven of her bodyguards at a checkpoint in a Shiite area of eastern Baghdad. Officials said she was traveling to Baghdad from her home in nearly Diyala province, a hotbed of sectarian violence, to attend a parliament session Sunday. One of her bodyguards escaped, officials said.

Al-Mashhadani's colleagues in the Iraqi Islamic Party, the largest Sunni political organization, blamed the ministers of interior and defense for failing to restore security in the capital. A party statement alleged that some of the kidnappers carried government-issue weapons.

A top party official, Ayad al-Sammaraie, said such kidnappings were aimed at undermining steps toward national reconciliation by fueling animosity among Iraqis so “they cannot sit together and talk.”

The U.S. Embassy condemned the kidnapping as an attempt to stoke sectarian tension and demanded her immediate release.

“We note that Ms. al-Mashhadani is an elected representative of the people of Iraq and that this act is repugnant to all who believe in the right of Iraqis to participate in their country's democracy,” the embassy said.

The latest violence occurred as al-Maliki, a Shiite, left for a tour of Persian Gulf countries to brief leaders on his plans for national reconciliation, including amnesty for Sunni Arab insurgents and efforts to heal the Shiite-Sunni rift.

Al-Maliki's first stop was Saudi Arabia, whose government fears that sectarian violence in Iraq could spill over into neighboring countries that are dominated by Sunnis but with large Shiite communities. He is also to visit Kuwait and the United Arab Emirates.

As part of the reconciliation plan, Iraqi and U.S. authorities Saturday freed 495 prisoners from U.S. jails across Iraq, completing a mass release announced by al-Maliki last month. Most of the prisoners were believed to be Sunnis.

“We are prepared in consultation with the Iraqi leaders to make future prisoner releases and to take other concrete steps to facilitate reconciliation,” U.S. Ambassador Zalmay Khalilzad said in a statement. “As the government moves forward with these steps, it can count on support from the U.S. government.”

U.S. and Iraqi officials had hoped that installation of a new unity government of Shiites, Sunnis and Kurds would stem the violence and encourage armed groups to join the political process. Those hopes were buoyed by the death of al-Zarqawi in an airstrike June 7.

However, more than 700 Iraqi civilians and security forces have died violently since al-Zarqawi's death, according to Associated Press figures.

Also Saturday, a U.S. soldier assigned to the 43rd Military Police Brigade was killed in a non-combat related accident, the military said, without providing further details.

Elsewhere, police found a grave in Baghdad containing bodies of at least six men who appeared to have been shot more than a month ago, Lt. Thaer Mahmoud said. The identities of the men were unknown, but their bodies were discovered in a Baghdad area notorious for sectarian killings.

Also Saturday, new details emerged into allegations that U.S. soldiers raped an Iraqi woman south of Baghdad in March, then killed her and three family members.

A U.S. official familiar with the investigation told the AP that the soldiers allegedly plotted the attack for about a week. The official, who spoke on condition of anonymity because the case is under investigation, said the soldiers doused the woman's body with flammable liquid and burned it to cover up the assault.

U.S. officials said they knew of the deaths but thought the victims died due to sectarian violence. But police Capt. Ihsan Abdul-Rahman said Iraqi officials received a report March 13 alleging that U.S. soldiers had killed the family.

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Associated Press writers Bassem Mroue, Sameer N. Yacoub and Bushra Juhi in Baghdad and Ryan Lenz in Beiji contributed to this report.

AP-ES-07-01-06 1418EDT