A car bomb exploded in a crowded outdoor market in Kirkuk, Iraq on Tuesday, killing at least 27 people, police said, a deadly reminder of the challenges facing the Iraqi government even as it celebrated the withdrawal of US combat troops from cities. The attack stained what had otherwise been a festive day as Iraqis commemorated the newly declared National Sovereignty Day with military parades and marching bands. It also came hours after four US soldiers were killed in combat Monday in Baghdad. Although there were no immediate claims of responsibility, the bombing and the way it was carried out bore the hallmarks of al-Qaida in Iraq. Despite the violence, Iraqi Prime Minister Nouri al-Maliki assured Iraqis that government forces taking control of urban areas were more than capable of ensuring security. "Those who think that Iraqis are not able to protect their country and that the withdrawal of foreign forces will create a security vacuum are committing a big mistake," he said in a nationally televised address. He later appeared at a military parade to mark the day in the walled-off Green Zone in central Baghdad, with soldiers and policemen marching in formation as Iraqi helicopters were flying overhead. The withdrawal, which was completed on Monday, was part of a US-Iraqi security pact and marks the first major step toward withdrawing all American forces from the country by Dec. 31, 2011. US President Barack Obama has said all combat troops will be gone by the end of August 2010. The car bomb exploded as the vegetable and poultry market was crowded with people shopping for their evening meal, police Brig. Gen. Sarhat Qadir said. Police and hospital officials gave the death toll and said about 40 people were wounded.