An Iraqi boy watches a U.S. tank roll by the wreckage of a car bomb in Baghdad. Two car bombs exploded minutes apart Sunday in a busy market in a mainly Shiite district, killing at least 18 people.

Baghdad, Iraq ? Cars, minibuses and roadside bombs exploded in Shiite Muslim enclaves across the city Sunday, killing at least 45 people in sectarian violence that defied the Baghdad security crackdown, while a radical anti-U.S. cleric raised a new threat to Iraq’s government.

Two officials close to Shiite cleric Muqtada al-Sadr said his followers would quit their six Cabinet posts today – a move that could leave Prime Minister Nouri al-Maliki’s already weak administration without enough support to stay in power.

And in a rare gesture of dissent from America’s partners in Baghdad, dozens of Iraqi policemen demonstrated in front of their station, accusing U.S. troops of treating them like “animals” and “slaves.”

The U.S. military command announced the combat deaths of three more Americans, bringing the toll to at least 3,300. Two British service members died when their helicopters collided in midair north of Baghdad.

Six powerful bombs, gunfire and artillery blasts enveloped Baghdad in a near-constant din that seemed a setback for the nine-week-old U.S.-Iraqi military campaign to pacify the capital.

U.S. commanders previously cited a slight decrease in violence since the crackdown began Feb. 14, but urged patience for what they warned would be a long, tough fight.

“Although we’re making steady progress … we have a long way to go,” Rear Adm. Mark Fox, a U.S. military spokesman in Baghdad, told reporters Sunday. “We will continue to face attacks from those who attempt to tear down what the Iraqi people have worked so hard to build.”

The crackdown is believed to have driven many insurgents from Baghdad, and violence has soared in areas outside the capital, such as the bombing in the Shiite holy city of Karbala that killed 47 people and wounded 224 Saturday.

But violence has spiked upward again in Baghdad, with Sunday’s six bombings coming just three days after a suicide bomber blew himself up inside parliament and killed a lawmaker.

“This week has been difficult for the Iraqi people,” Fox acknowledged.