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.
'Sick souls'
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.
[The bombing] scarred the beautiful face of Baghdad by destroying the religious landmarks it has known over the centuries
Prime Minister Nouri Maliki
In pictures: Baghdad blast
"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.
Night-time assault
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.
Al menos 78 personas murieron y 218 han resultado heridas tras una explosión de un auto-bomba, según reportaron oficiales. El ataque se perpetró cerca la mezquita chií Julani.

La detonación tuvo lugar en una Plaza Khalani cerca de las 2 p.m., un área densamente transitada. La explosión causó que 20 vehículos fueran destruidos, causando que una columna de humo ascendiera al cielo opacando lo que había sido un buen día.

El Primer Ministro de Iraq, Nuri Al Maliki, condenó la violencia y culpó a Sadamistas y otros grupos fundamentalistas por la explosión y la reciente ola de ataques terroristas a centros religiosos.

El área comercial de Sinak, donde se llevó a cabo la acción terrorista, había sido víctima de un ataque el 28 de mayo.

La bomba detonó interrumpiendo la misa de mediodía de cientos de seguidores de la religión Islam. La explosión daño severamente una de las paredes del templo, dejando un cráter de aproximadamente seis metros de diámetro y tres metros de profundidad.

Seis de los fallecidos habitaban hogares cercanos a la mezquita antes de que ésta colapsara, y alrededor de 25 puestos de comercio fueron dañados potencialmente.

Desde la destrucción de la mezquita de Samarra, la más venerada en Iraq, que tuvo lugar el 22 de febrero de 2006, destruyera la cúpula dorada del templo y luego éste colapsara, las mezquitas y sectas iraquíes han sido víctimas de múltiples ataques terroristas.