BAGHDAD - 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-US cleric raised a new threat to Iraq's government.

Iraqi policeman stands in front of a mini bus destroyed after a suicide bomber blew himself up in northwest Baghdad, Iraq, Sunday, April 15, 2007. [AP]

Two officials close to Shiite cleric Muqtada al-Sadr said his followers would quit their six Cabinet posts Monday - 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 US troops of treating them like "animals" and "slaves."

The US military command announced the combat deaths of three more Americans. Two British service members died when their helicopters crashed in midair north of Baghdad, and hours later a US helicopter was hit by ground fire near Mosul but landed safely with no injuries.

Six powerful bombs, gunfire and artillery blasts enveloped Baghdad in a near-constant din that seemed a setback for the 9-week-old US-Iraqi military campaign to pacify the capital.

US 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 US 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.

The carnage caused some to voice doubts about the Baghdad crackdown.

"The security plan has made more troubles for Iraqis than helping them," said Juma'a Khamis, 42, a technician who lives in the capital. "There have been no positive results. It's a failure, and so is the government."

Others retained hope that the campaign could carve out breathing room for Iraqi forces to regain control of the city.

"We need to build our security forces, and step by step we can achieve stability," said Nassir Amir, a 31-year-old civil engineer. "The Iraqi government is trying its best but it faces a lot of difficulties ... It needs more time - maybe one year at least."

Violence isn't the only threat to the government. A pullout by the al-Sadr faction, which provided the crucial votes that put al-Maliki in office, could collapse his already shaky regime.

Al-Sadr's six followers on the 37-seat Cabinet would officially withdraw from the government Monday, said Saleh al-Aujaili and Hassan al-Rubaie, members of the Sadrist bloc in parliament. They said al-Sadr's 30 legislators would stay in parliament.

The officials said al-Sadr ordered the Cabinet ministers to quit in protest over the arrests of leaders in his Shiite militia during the Baghdad crackdown and for the prime minister's failure to back setting a timetable for US withdrawal.

Earlier in the day, dozens of Iraqi police officers chanted "No, no to America! Get out occupiers!" during a protest at the Rashad station in Baghdad's eastern neighborhood of Mashtal. US troops in two Humvees and a Bradley fighting vehicle watched from a distance.

Officers complained that American troops do not treat them with respect, but it wasn't clear if any specific incident set off the demonstration.

The day's first bombings came at midday, when two cars packed with explosives blew up five minutes apart in a busy market area of Shurta Rabia, a mostly Shiite area of western Baghdad.

The first detonated in front of a kebab restaurant, and the second went off as rescuers were evacuating victims, police said. Many women and children were reported among the 18 dead and 50 wounded.

Less than two hours later, a suicide bomber blew himself up on a minibus near a courthouse in the mainly Shiite neighborhood of al-Utafiyah of northwest Baghdad - killing at least eight people and wounding 11, officials said. Many victims were severely burned, an official at Khazimiyah Hospital said.

Another minibus rigged with explosives detonated near a cluster of electronics shops in the predominantly Shiite district of Karradah, killing 11 people and wounding 15, authorities said.

The owner of a glass shop said he saw someone park the bus and leave.

"It was an ordinary thing because usually bus drivers stop there waiting for passengers, so we didn't suspect anything," said the witness, who gave only his nickname, Abu Jassim. "Five minutes later, the bus blew up - damaging the surrounded area and burning more than eight civilian cars that were passing by."

After nightfall, insurgents struck in Karradah again - with two roadside bombs exploding within five minutes and 20 yards of one another. Eight more people died and 23 were wounded, police said. Six shops and several parked cars were damaged.

The two British helicopters crashed after colliding over a rural area near Taji, 12 miles north of Baghdad. Two British crew members were killed and four were injured, one very seriously.

"We believe it's almost certainly an accident," Prime Minister Tony Blair told British Broadcasting Corp. television.

British forces, headquartered in the southern city of Basra, rarely fly missions near Taji.

"I can't talk about the particular mission they were involved in, but we do have units operating as part of the coalition across Iraq," a British defense official said on condition of anonymity, in line with government policy.

In the northern city of Mosul, a US Kiowa helicopter was hit by insurgent ground fire, causing it to make a "precautionary landing due to mechanical problems," a military statement said. The aircraft was assisting Iraqi soldiers whose outpost was attacked by insurgents, it said.

Uninjured, both pilots "returned to their base, launched another helicopter and went back to the engagement area to continue the fight," the statement said. Four Iraqi soldiers were killed in the battle, it said.

One US Army soldier was killed by small arms fire Sunday while trying to reach an Iraqi police unit under attack near a mosque in southern Baghdad, the military said in a statement. One civilian was wounded in the incident.

Another soldier died Saturday when a roadside bomb exploded alongside a foot patrol south of Baghdad, the military said. A Marine died the same day in combat in Anbar province, west of the capital.

A third soldier died from non-combat related causes Saturday while on leave in Qatar, the military said.