Bombings and shootings killed at least 33 people in Baghdad and surrounding areas on Monday, including a group of high school students on a bus headed for final exams, as violence intensified before a planned withdrawal next week of U.S. troops from urban areas.

The bombings, nearly all in Shiite areas of the capital, came just two days after the year's deadliest attack — a truck bombing that killed at least 75 people in northern Iraq.

Overall violence has declined drastically over the past two years, but the recent attacks have raised concerns about the Shiite-dominated government's ability to provide security around the country without the immediate help of the U.S. troops remaining in Iraq. More than 100 people have died in three days of bloodletting, mostly from bombings but also from shootings.

Starting June 30, most of the 133,000 American troops left here will be housed in large bases outside the capital and other cities — unable to react unless called on for help. The withdrawal is part of an agreement under which all U.S. troops are to leave Iraq by the end of 2011.

The reclusive Shiite cleric Muqtada al-Sadr called on the Shiite-led government to take whatever steps necessary to protect Iraqis from attacks. But in a statement, the anti-American cleric blamed the violence on the continued presence of U.S. troops in the country and demanded a faster withdrawal.

"The Iraqi people are heading toward a new phase that might lift them out from their suffering," the cleric said in a statement. He also called on his followers to remain peaceful.

Last August, he ordered militiamen of his Mahdi Army to lay down their arms and take up social work. The edict came just after U.S. troops working with Iraqi soldiers routed the militia in its stronghold in Baghdad's Sadr City.

In that Shiite bastion, a roadside bomb exploded next to a bus carrying high school students to their final exams on Monday, killing at least three people and wounding 13, including three of the students, police said. The bomb peppered the bus with shrapnel and left the floor of the vehicle littered with blood-soaked textbooks.

The U.S. military said only one civilian was killed and eight wounded. Conflicting casualty tolls are common in the aftermath of bombings in Iraq as victims are often taken to multiple hospitals.

At least five people also were killed and 20 were wounded by a bomb planted near a car in the central Karradah district of the Iraqi capital, on the east side of the Tigris River. The bomb exploded on a road leading to a checkpoint that controls access to a bridge into the Green Zone, which houses the Iraqi government and U.S. Embassy.

The U.S. military put the casualty toll at two killed and six wounded.

Another roadside bomb targeted a police patrol in a commercial area of eastern Baghdad's Ur district, killing three and wounding 25, according to police, although the U.S. military said just two were killed.

A suicide car bomber then targeted the mayor's offices in Abu Ghraib, a predominantly Sunni district west of Baghdad.

The explosion occurred when the car struck a civilian vehicle before reaching the government building, damaging a nearby U.S. vehicle that was providing security for a meeting, said Maj. David Shoupe, a spokesman for U.S. forces in Baghdad.

He said four civilians were killed and 10 people were wounded, including three U.S. soldiers, while a local police officer said seven civilians were killed and 13 wounded.

North of the capital, a roadside bomb struck an Iraqi army patrol, killing three Iraqi soldiers near Khanaqin, near the Iranian border, according to the security headquarters in Diyala province.

And north of Baghdad, a parked motorcycle loaded with explosives blew up in an open-air public market in the predominantly Shiite area of in Husseiniya, killing five people and wounding another 22, police and hospital officials said.

Gunmen also killed at least seven people in separate attacks in the northern city of Mosul, including a woman and four Iraqi security forces, according to separate police reports.

The Iraqi officials all spoke on condition of anonymity because they weren't authorized to release the information.

In northern Iraq, rescue crews were searching for at least 12 people still missing in a massive explosion Saturday near the ethnically tense city of Kirkuk that flattened a Shiite mosque and dozens of mud-brick houses around it.

Iraqi police have blamed Al Qaeda in Iraq for the attack, saying it was part of an insurgent campaign to destabilize the country and undermine confidence in the government.

Americans will remain ready to help, as they were in the aftermath of Saturday's bombing, but many Iraqis fear their departure after two years of a steady urban presence will prove deadly