BAGHDAD — Iraqi security forces bolstered checkpoints and banned motorcycles from the streets of Baghdad as they prepared Sunday for more violence before this week’s withdrawal of U.S. combat troops from the capital and other cities and towns.

Despite the increased checks, a roadside bomb targeting a U.S. convoy in eastern Baghdad wounded six bystanders. A car bomb also exploded at a police academy in western Baghdad, killing one police officer and wounding six others, police said.

Insurgents apparently took advantage of a major sandstorm Sunday that reduced visibility to just a few yards in some places.

Iraq’s main Sunni political bloc joined Shiite Prime Minister Nouri al-Maliki in describing the Tuesday’s deadline for the U.S. withdrawal from urban areas as a turning point for the country.

Al-Maliki’s government has declared Tuesday as National Sovereignty Day and decreed a public holiday.

“June 30 is an important turning point on the civilian, security and political levels, and this is the feeling shared by all Iraqis,” Salim al-Jubouri, spokesman for the Sunni Iraqi Accordance Front, said in a statement.

He also said “terrorist elements” would try to disrupt the withdrawal but added that Iraqi forces were capable of ensuring security. Although considerable progress has been made at reconciling Sunnis and Shiites, there is still a divide between the two Islamic sects that nearly brought the country to the brink of civil war in 2006-07.

Police banned all motorcycles from Baghdad’s streets until further notice after motorcycles were used last week in three separate attacks that killed more than 100 people. More than 250 people have been killed since June 20 in a spate of bombings.

Iraqi officials have warned people to stay away from crowded places.

Iraqi officials have blamed al-Qaida in Iraq for recent attacks, and the U.S. military believes the terrorist group is struggling to regain a foothold after being beaten back over the past two years. U.S. military officials believe the group has plunged from thousands at its peak in 2006-2007 to hundreds now.

Over the weekend, few if any of the more than 130,000 U.S. troops still in Iraq were visible in its cities, as most already pulled out of urban centers in recent weeks. They have assembled in large bases outside urban centers and will continue to conduct combat operations in rural areas and near the border.

“They’ve been working towards this for a long time. And security remains good,” said the top U.S. commander in Iraq, Gen. Ray Odierno. “We’ve seen constant improvement in the security force, we’ve seen constant improvement in governance. And I believe this is the time for us to move out of the cities and for them to take ultimate responsibility.”