Four large bombs exploded in mostly Shiite areas of Baghdad on Wednesday, killing at least 160 people and wounding scores as violence climbed toward levels seen before the US-Iraqi campaign to pacify the capital began two months ago. In the deadliest of the attacks, a parked car bomb detonated in a crowd of workers at the Sadriyah market in central Baghdad, killing at least 112 people and wounding 115, said Raad Muhsin, an official at Al-Kindi Hospital where the victims were taken. A police official confirmed the toll, speaking on condition of anonymity because he was not authorized to release the information. Among the dead were several construction workers who had been rebuilding the mostly Shiite marketplace after a bombing destroyed many shops and killed 137 people there in February, the police official said. The laborers typically finish work around 4 p.m. each day. One of those wounded, 28-year-old Salih Mustafa, said he was waiting for a minibus to head home when the blast went off at 4:05 p.m. "I rushed with others to give a hand and help the victims," he said. "I saw three bodies in a wooden cart, and civilian cars were helping to transfer the victims. It was really a horrible scene." The market is situated on a side street lined with shops and vendors selling produce, meat and other staples. It is also about 500 meters (yards) from a Sunni shrine. About an hour earlier, a suicide car bomber crashed into an Iraqi police checkpoint at an entrance to Sadr City, the capital's biggest Shiite Muslim neighborhood and a stronghold for the militia led by radical anti-US cleric Muqtada al-Sadr. The explosion killed at least 33 people, including five Iraqi security officers, and wounded 45, police and hospital officials said. Black smoke billowed from a jumble of at least eight incinerated vehicles that were in a jam of cars stopped at the checkpoint. Bystanders scrambled over twisted metal to drag victims from the smoldering wreckage as Iraqi guards staggered around stunned. Earlier, a parked car exploded near a private hospital in the central neighborhood of Karradah, killing 11 people and wounding 13, police said. The blast damaged the Abdul-Majid hospital and other nearby buildings. The fourth explosion was from a bomb left on a minibus in the central Rusafi area, area, killing four people and wounding six others, police said. Also in Baghdad, four policemen were killed Wednesday afternoon when gunmen ambushed their patrol south of the city center, police said. Six pedestrians were wounded in the gunfire. US officials had cited a slight decrease in sectarian killings in Baghdad since the US-Iraqi crackdown was launched Feb. 14. But the past week has seen several spectacular attacks on the capital, including a suicide bombing inside parliament and a powerful blast that collapsed a landmark bridge across the Tigris River. "We've seen both inspiring progress and too much evidence that we still face many grave challenges," Maj. Gen. William Caldwell, a US military spokesman, told reporters Wednesday. "We've always said securing Baghdad would not be easy." Meanwhile, to the west of the city, US troops killed five suspected insurgents and captured 30 others in a raid in Anbar province, a day after police uncovered 17 decomposing corpses beneath two school yards in the provincial capital. The raid took place early Wednesday near Karmah, a town northeast of Fallujah in Anbar, which has been a stronghold for Sunni insurgents. American forces raided a group of buildings suspected of being used by militants and found explosives inside one of them, the military said in a statement. A helicopter was called in and dropped precision-guided bombs on the buildings, it said. The soldiers came under fire and shot back, killing five Iraqis and wounding four others, the statement said. The wounded were taken to a military hospital and remained in US custody. Twenty-six other people were detained as well, the military said. The bodies found a day earlier at school yards in Ramadi, Anbar's provincial capital, were discovered after students and teachers returned to the schools a week ago and noticed an increasingly putrid odor and stray dogs digging in the area, police Maj. Laith al-Dulaimi said. Ramadi had been a stronghold of Sunni insurgents and al-Qaida fighters until recently, when US forces in the region and the Iraqi government successfully negotiated with many local tribal leaders to split them off from the more militant insurgent groups. The US military also reported that a suspected insurgent was killed and eight captured in two raids north of Baghdad on Wednesday. Some of the suspects were believed linked to al-Qaida in Iraq and to a militant cell that has used chlorine in car bombings, the statement said. Separately, US officials announced that last week they found 3,000 gallons of nitric acid hidden in a warehouse in downtown Baghdad. US forces discovered the acid, a key fertilizer component that can also be used in explosives, during a routine search Thursday, the military said. Iraqi troops also took charge of security Wednesday in the southern province of Maysan, a region that borders Iran and the fourth province to come under full Iraqi security control since the 2003 US invasion. A ceremony was held in Maysan's provincial capital of Amarah, 320 kilometers (200 miles) southeast of Baghdad, and was attended by senior Iraqi and coalition officials including Iraqi National Security Adviser Mowaffak al-Rubaie and the British commander in southern Iraq, Maj. Gen. Jonathan Shaw. Al-Rubaie said that in order for a timetable to be set for the withdrawal of foreign troops, Iraqi forces and local authorities have to be ready to take over. He was apparently referring to calls by some Sunni Arab groups and al-Sadr's Shiite followers to set a timetable for a pullout. "We should work to create these circumstances in all provinces, in order to revert security to Iraqis and end the foreign presence," said al-Rubaie, who represented Prime Minister Nouri al-Maliki, the commander in chief of Iraq's armed forces. Al-Maliki was supposed to attend the ceremony but his trip was canceled without explanation.