A policeman examines the site of a car bomb attack in Karrada District, central Baghdad on Monday. A parked car bomb killed five people and wounded 25 in Karrada in the city centre, police said. (Bassim Shati/Reuters)

At least 21 people were killed in bombings in the Baghdad area Monday as violence escalated ahead of the United States's plan to withdraw its troops from Iraq's urban areas next week.

Nearly all of the bombings happened in Shia areas of the capital, and will test the Shia-dominated government's ability to provide security without the immediate help of 130,000 U.S. troops.

All U.S. troops are scheduled to leave Iraq by the end of 2011, and starting June 30, most of them will be housed in bases outside the capital and other cities — unable to react unless called on for help.

The reclusive and anti-American Shia cleric Muqtada al-Sadr called on the government to take whatever steps necessary to protect Iraqis from attacks. In a statement, he 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.

Monday's bombings came just two days after the year's deadliest attack, a truck bombing that killed at least 75 people in northern Iraq.

The latest attacks included:

• A roadside bomb that exploded next to a bus carrying high school students to their final exams.
• A bomb that 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.
• A roadside bomb that targeted a police patrol in a commercial area of eastern Baghdad's Ur district.
• A suicide car bomber who targeted the mayor's offices in Abu Ghraib, a predominantly Sunni district west of Baghdad.

With files from The Associated Press