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. It was the deadliest attack since the new national unity government took office six weeks ago and one of the biggest this year.

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.

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 private cars and rushed them to hospitals. Late Saturday, Iraqi state television reported that 66 people died and about 100 were wounded.

The bombing was the deadliest since an April 7 suicide attack that killed 87 people at a Shiite mosque in Baghdad.

A statement posted Saturday on the Internet claimed responsibility for the Sadr City blast in the name of the “Sunna Supporters Group” in retaliation for Shiite attacks on Sunnis. The statement, whose authenticity could not be verified, accused Shiites of killing Sunnis and raping Sunni women in detention.