BAGHDAD  A bomb placed in a motorbike exploded at an outdoor market in Baghdad on Friday, killing about a dozen people and wounding scores. It marked the third straight day of violence in the capital before the Tuesday deadline for American combat troops to withdraw from Iraqi cities.

After another similar bombing outside a billiard hall in Baghdad later on Friday that killed two people and wounded seven, the authorities ordered all motorbikes off the streets indefinitely.

Nearly 200 people were killed and hundreds were wounded in attacks over the past week in Baghdad and elsewhere in the country, with the deadliest attacks aimed at Shiites. The violence has raised fears of a new bout of sectarian warfare of the sort that ripped Iraq apart in 2005, and that could lead Iraqis once again to seek the protection of militias and armed groups instead of government forces.

The reaction by Shiite leaders echoed those concerns.

The bombing on Friday ripped through a market for new and secondhand motorbikes held each week under a bridge in the working-class neighborhood of Nahdha in central Baghdad.