At least 130 people have been killed and 305 injured in a lorry bombing at a market place in central Baghdad, Iraqi security officials have said.
Nearby hospitals were overflowing with dead and injured
Police say the vehicle blew up in al-Sadriya district as people bought food ahead of a night-time curfew.
It shattered stalls and left a huge crater in the street, in the heavily populated, predominantly Shia area.
Only the Sadr City bombings in November, which killed more than 200 people, have claimed more lives.
Iraqi Prime Minister Nouri Maliki blamed loyalists of the executed former leader Saddam Hussein for the al-Sadriya attack.
"The Iraqi people and the world is shocked by this... The Saddamists have returned to commit a new crime," he said.
"We reassure the population that we will put an end to these crimes."
The White House in Washington called it another atrocity aimed at innocent people.
Maj Gen Jihad al-Jaberi of the interior ministry told state television the lorry was carrying one tonne of explosives that were detonated by a suicide bomber.
Rescuers pulled the dead and injured from the rubble of the blast and loaded them on to pick-up trucks to be ferried to hospital.
Wards and corridors overflowed at the nearby Ibn al-Nafis hospital, with relatives screaming for help for their loved ones.
One injured man at Ibn al-Nafis told Reuters news agency: "I was in my shop and there was a great explosion and the roof fell in on me. I woke up here in hospital."
The same market was hit by car bombs on 2 December that killed more than 50 people.
Another attack, on the Haraj market in Baghdad on 22 January, killed 88 people.
BBC world affairs correspondent Mike Wooldridge says the attacks on markets are widely seen as militant groups deliberately stepping up their activities before a new joint Iraqi-US security operation for Baghdad gets under way.
The US is to deploy an extra 21,500 troops.
On Saturday, Iraq's most prominent Shia cleric, Ayatollah Ali al-Sistani, made a new appeal to all Iraqis.
"Everybody knows the necessity for us to stand together and reject the sectarian tension to avoid stirring sectarian differences," he said.
Seven bombs also went off in Kirkuk in the north
Earlier, seven car bombs in the ethnically mixed northern city of Kirkuk killed five people and injured 40.
Two of the bombs targeted the headquarters of two Kurdish parties - the Kurdish Democratic Party led by of Massoud Barzani, head of the northern Kurdish region, and the Patriotic Union of Kurdistan, headed by Iraq's President, Jalal Talabani.
Razqar Ali, a Kurdish leader and head of Kirkuk provincial council, accused militants of trying to destabilise the city amid efforts by some Kurds to include it in the autonomous Kurdish region.
A curfew was imposed after the blasts and would run from 1600 (1300 GMT) to 0600 on Sunday, AFP news agency said.
Earlier, police said gunmen attacked a checkpoint near Samarra, killing six police and injuring another six.
Samarra is a mainly Sunni town 125km (80 miles) north of Baghdad where an attack on an important Shia shrine last February sparked Iraq's current sectarian violence.