Chaotic scenes as people tried to put out fires and help the wounded
At least 27 people have been killed by a car bomb at a market in the northern Iraqi city of Kirkuk, officials say.
The attack in the Shurja district came as Iraqis celebrated the withdrawal of US troops from towns and cities in Iraq, six years after the invasion.
US President Barack Obama said the move was an important milestone for Iraq, but that "difficult days" lay ahead.
The Kirkuk blast came 10 days after a truck bomb killed more than 70 in the city's deadliest attack in over a year.
Iraqi and US troops have been on alert for attacks during the pullback, which was declared a national holiday.
Police Brig Gen Sarhat Qadir told the Associated Press news agency that at least 40 people had been wounded in the latest blast, caused by an explosives-laden vehicle parked near the crowded outdoor Shurja market.
A teeming maze of shops and stalls, Shurja is one of the country's best-known markets, attracting buyers and sellers from all over Iraq, say correspondents.
US President Obama: "There will be difficult days ahead"
Kirkuk, about 250km (155 miles) from Baghdad, was also the scene of two suicide bombings last month, in which 14 people were killed.
The city is the centre of northern Iraq's oil industry, and home to a volatile mix of Kurds, Arabs, Christians and members of the Turkmen community.
The BBC's Jim Muir in Baghdad says Tuesday's car bomb appears to be just the kind of attack designed to stir up ethnic tensions between Kurds and Arabs.
Most of the other bombs that have killed around 250 people in the past fortnight have been aimed at Shia areas.
Our correspondent says the clear aim is to reignite the sectarian carnage that took the country to the brink of civil war three years ago.
With American troops now taking a back seat, the big question, our correspondent adds, is can Iraqi forces cope with the challenge?
Iraqi Prime Minister Nouri Maliki is adamant they can.
Those who think Iraqis are unable to protect their country commit a big mistake
"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 earlier, in a nationally televised address.
Our correspondent says Mr Maliki's police and troops will have to prove on the ground that they are Iraqis - not Shias or Sunnis or Kurds - if they are to prevail.
In Washington, President Obama called the Kirkuk bombing "senseless", adding: "The future belongs to those who build, not to those who destroy."
Meanwhile, despite their pullback from cities and towns, US troops will still be embedded with Iraqi forces.
On Monday, four US soldiers were killed in combat in Baghdad.
US commanders have said security and stability are improving.
Iraqi soldiers paraded through Baghdad's streets on Monday in vehicles decorated with flowers and Iraqi flags, while patriotic songs were played through loudspeakers at checkpoints.
The pullback comes two years after the US "surge" of extra troops between February and June 2007, which saw US troop levels in Iraq reach about 170,000.
US-led combat operations are due to end by September 2010, with all troops gone from Iraq by the end of 2011.
Some 131,000 US troops remain in Iraq, including 12 combat brigades, and the total is not expected to drop below 128,000 until after the Iraqi national election in January.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.