A car bomb has killed at least 27 people in a northern Iraqi city, on the day that Iraqi forces officially assumed control of security in Baghdad and other urban areas.
At least 45 other people were wounded when the car bomb exploded at a market in Kirkuk, leaving smoldering rubble where people had been shopping for food and other goods.
The oil-rich Kirkuk region is at the center of a power struggle between ethnic and religious groups, including Kurds, Arabs, Turkmen, Muslims and Christians.
The bombing marred a day of celebrations. Earlier Tuesday in Baghdad, Iraqi soldiers and police staged a long parade in full dress uniform. Prime Minister Nouri al-Maliki called the day a "great victory."
In Washington, U.S. President Barack Obama hailed the withdrawal of U.S. combat troops from urban areas as an important milestone and an important step forward for Iraq.
He added that with progress comes responsibility, and he warned that violence will continue.
President Obama also praised the service and sacrifice of U.S. troops who have served in Iraq.
The Iraqi government had declared Tuesday a "Day of National Sovereignty" to mark the transfer of control.
While some Iraqis are joyous at seeing U.S. combat troops leave, others are nervous about the change in security and fear an increase in militant attacks.
The U.S. commander in Iraq, General Raymond Odierno, spoke to journalists at the U.S. Department of Defense from Baghdad via video-conference Tuesday. He declined to give a figure for the number of non-combat troops that remain in Iraqi cities to train and advise Iraqi forces, saying the number will change daily depending on need.
General Odierno emphasized that the figure is significantly lower than the number of troops that have been in the cities until now.
The withdrawal is part of a U.S.-Iraq security deal that sets a timeline for the pullout of all U.S. forces by the end of 2011. At present, about 130,000 U.S. troops remain in the country to conduct combat duties outside cities and to advise Iraqi forces within cities.
Some information for this report was provided by AFP and Reuters.