At least 78 people have been killed and another 218 injured in a truck bomb attack near a Shia mosque in the centre of the Iraqi capital, Baghdad.
The prayer hall of the al-Khilani mosque was destroyed
The massive explosion in the busy Sinak commercial district destroyed part of the al-Khilani mosque and sent large clouds of black smoke into the air.
It was the latest in a series of recent attacks on Shia and Sunni mosques.
The bombing came as US forces began a major offensive against militants linked to al-Qaeda north of Baghdad.
Around 10,000 troops are taking part in Operation Arrowhead Ripper in and around the city of Baquba in Diyala province, a centre of the Sunni Arab insurgency.
The US military says 22 militants have been killed, but local officials say there have also been civilian deaths.
The explosion in Baghdad rocked the centre of the city and sent a big cloud of black smoke into the sky.
The bomb, reportedly packed into a truck parked outside the al-Khilani mosque, went off at a particularly busy time of the day. There were traffic jams leading up to a nearby checkpoint.
Witnesses said the effect was devastating, with the mosque's prayer hall being destroyed and several vehicles being set on fire.
The mosque's imam, Sheikh Saleh al-Haidari, said the blast had hit worshippers as they left afternoon prayers.
"This attack was planned and carried out by sick souls, damaging the mosque's outer wall and collapsing my office and the room above it," he told the Associated Press.
"There are number of bodies being pulled from the rubble and a number of worshippers were killed or injured."
Prime Minister Nouri Maliki condemned the bombing and called for national unity.
"Once again the groups of conspiracy, terror and takfir [declaring people infidels] have violated the sanctity of houses of God and spilled the blood of innocent people," he said.
"[The bombing] scarred the beautiful face of Baghdad by destroying the religious landmarks it has known over the centuries."
The attack was the bloodiest in Baghdad since a four-day curfew was lifted on Sunday following the bombing of the al-Askari shrine in Samarra, one of the most revered sites in Shia Islam.
Since then, there have been a number of reprisal explosions at Sunni mosques in Baghdad and the south.
North of Baghdad, US and Iraqi troops are continuing to conduct a major offensive to drive out militants linked to al-Qaeda.
Operation Arrowhead Ripper started overnight on Tuesday with a ground and air assault to secure the city of Baquba, a centre of the Sunni Arab insurgency.
The operation's commander, US Brig-Gen Mick Bednarek, told the BBC that the aim was to eliminate the insurgents and bring them to justice.
He said the US military did not want to simply push them to another part of Iraq.
"We see the insurgency migrate," he told the World Service Newshour programme.
"Many of them have come to Diyala province and the acts of violence - horrific, terrible events against the Iraqi people, their families, security forces etc here in the province - warranted an increase in combat operations and focus by both the Iraqi security forces and the coalition."
In other violence, police in the southern city of Nasiriya said at least 35 people were killed during two days of fighting between Iraqi security forces and Shia militiamen loyal to the cleric Moqtada Sadr.