At least 64 people have been killed by a suicide truck bomb near the northern Iraqi city of Kirkuk, Iraqi police say.
About 160 others were injured in the blast, which happened near a Shia mosque in Taza, officials said.
At least a dozen nearby mud-brick homes were flattened by the explosion, and the mosque also was badly damaged.
The latest attack comes days before US forces are due to withdraw from towns and cities in Iraq, leading to concerns that violence could escalate.
"This ugly crime is an attempt to harm security and stability and spread mistrust of the Iraqi forces," Iraqi Prime Minister Nouri Maliki said in a statement.
Just hours before the attack, he had promised the withdrawal would go ahead as promised, calling it a "great victory".
"Don't lose heart if a breach of security occurs here or there," he said.
The bombing is one of the deadliest attacks in Iraq so far this year, says the BBC's Jim Muir in Baghdad.
It happened as worshippers were leaving the packed Al-Rasoul mosque, run by the minority Turkmen community in the town of Taza, just to the south of Kirkuk, after midday prayers.
The force of the blast left a deep crater in the ground.
Victims were ferried to Kirkuk's main Azadi Hospital, where there were chaotic scenes as bloodied casualties, including children, were rushed into wards.
Kirkuk, about 250km (155 miles) from Baghdad, was 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 US plans to withdraw its troops from Iraqi cities and major towns by 30 June, and is due to end combat operations across Iraq by September 2010, leaving Iraqi security forces to cope alone.
There are concerns that insurgents may try to take advantage of the withdrawal, although the country's leaders say Iraqi forces are capable of handling internal security without US support.