Two nearly simultaneous bombs struck a predominantly Shi'ite commercial area in central Baghdad, killing at least 78 people and wounding at least 156, said Deputy Health Minister Hakim al-Zamili. The first blast occurred shortly after noon when a bomb left in a bag placed among the stalls of vendors selling DVDs and secondhand clothes exploded in the Bab al-Sharqi area between Tayaran and Tahrir squares. It was followed almost immediately by a parked car bomb just meters away. The explosions came hours after gunmen killed a female teacher as she was on her way to work at a girls' school in the mainly Sunni area of Khadra in western Baghdad, police said, adding that the teacher's driver was wounded in the drive-by shooting. On Sunday, two US Marines were killed in separate attacks in the Anbar province, an insurgent stronghold west of Baghdad, the military said. The deaths came a after 25 US troops were killed Saturday in the third-deadliest day since the war started in March 2003, eclipsed only by the one-day toll 37 US fatalities on January 26, 2005, and 28 on the third day of the US invasion. The heaviest tolls on Saturday came from a Black Hawk helicopter crash in which 12 US soldiers were killed northeast of Baghdad as well as an attack on a provincial government building in the Shi'ite holy city of Karbala that left five US troops dead. The violence underscores the challenges faced by US and Iraqi forces as they seek to rein in Sunni insurgents and Shi'ite militias that have made the capital and surrounding areas a battleground. Iraq's prime minister has dropped his protection of an anti-American cleric's Shi'ite militia after US intelligence convinced him the group was infiltrated by death squads, two officials said. In a desperate bid to fend off an all-out American offensive, the radical cleric Muqtada al-Sadr last Friday ordered the 30 lawmakers and six Cabinet ministers under his control to end their nearly two-month boycott of the government. They were back at their jobs Sunday. Al-Sadr had already ordered his militia fighters not to display their weapons. They have not, however, ceded control of the formerly mixed neighborhoods they have captured, killing Sunnis or forcing them to abandon their homes and businesses. Prime Minister Nouri al-Maliki's turnaround on the Mahdi Army was puzzling because as late as October 31, he had intervened to end a US blockade of Sadr City, the northeast Shi'ite enclave in Baghdad that is headquarters to the militia. It is held responsible for much of the sectarian bloodshed that has turned the capital into a battle zone over the past year. Shi'ite militias began taking revenge after more than two years of incessant bomb and shooting attacks by Sunni insurgents. Sometime between then and Nov. 30, when the prime minister met US President George W. Bush, al-Maliki was convinced of the truth of American intelligence reports which contended, among other things, that his protection of al-Sadr's militia was isolating him in the Arab world and among moderates at home, the two government officials said. "Al-Maliki realized he couldn't keep defending the Mahdi Army because of the information and evidence that the armed group was taking part in the killings, displacing people and violating the state's sovereignty," said one official. Both he and a second government official who confirmed the account refused to be identified by name because the information was confidential. Both officials are intimately aware of the prime minister's thinking. "The Americans don't act on rumors but on accurate intelligence. There are many intelligence agencies acting on the ground, and they know what's going on," said the second official, confirming the Americans had given al-Maliki overwhelming evidence about the Mahdi Army's deep involvement in the sectarian slaughter. Earlier this month, Bush and al-Maliki separately announced a new security drive to clamp off the sectarian violence that has riven the capital and surrounding regions. Bush announced an additional 21,500 American soldiers would be sent to accomplish the task and al-Maliki has promised a similar number of forces, who will take the lead in the overall operation. Iraq's Special Forces Command division has already teamed with the Americans since late last year for a series of pinpoint attacks in which at least five top Mahdi Army figures have been killed or captured. The neighborhood-by-neighborhood sweep, expected to begin in earnest by the first of the month, will target Sunni insurgents, al-Qaida in Iraq and its allied militant bands equally with Shi'ite militias, both the Mahdi Army and the Badr Brigade. The latter is the Iranian-trained military wing of Iraq's most power Shi'ite political group, the Supreme Council for the Islamic Revolution in Iraq. The first government official said al-Maliki's message was blunt. "He told the sheik that the activities of both the Sadrist politicians and the militia have inflamed hatred among neighboring Sunni Arab states that have been complaining bitterly to the Americans," the official said. Sunni Muslims are the majority sect in key Arab countries like Saudi Arabia, Jordan and Egypt, all of which have shunned al-Maliki. Shi'ites were long oppressed by Iraqi's Sunni minority and vaulted to power with the ouster of Saddam Hussein. Many of the leading Shi'ite figures in Iraq have deep historical ties to Iran, also a majority Shi'ite state, whose growing muscle in the Middle East is deeply threatening to the autocratic Sunni regimes in the region. As the Saturday death toll among American troops was mounting, the military reported five soldiers had been killed in an attack on a security meeting in provincial government building in Karbala, south of the capital. Thousands of pilgrims have arrived in the holy city to mark Ashoura, the festival at the start of the Islamic new year that marks the death of Imam Hussein, grandson of the Prophet Muhammad and one of the most-revered Shi'ite saints. Iraqi officials said on Sunday that the gunmen who attacked the meeting wore military uniforms and arrived in black sport utility vehicles commonly used by foreign dignitaries, an apparent attempt to impersonate American forces.