Bombs and mortar shells struck Sadr City at 15-minute intervals, beginning about 3 p.m. with the first bombing hitting a vegetable market. Shiites responded almost immediately, firing 10 mortar rounds at the holiest Sunni shrine in Baghdad, the Abu Hanifa Sunni mosque in Azamiya.

Speaking in a televised broadcast, Iraqi Prime Minister Nouri Maliki appealed for restraint after the deadliest single strike in Iraq since the war began more than three years ago.

"We denounce sectarian practices that aim to destroy the unity of the nation," Mr Maliki said.

The Iraqi authorities put Baghdad's seven million residents under curfew in the evening, saying all people and vehicles must stay off the streets until further notice. Baghdad's airport and Basra's air and sea ports in the south have been closed.

Leaders of Iraq's Shiite, Sunni and Kurdish communities also held a joint news conference in which they appealed for calm. Iraq's most prominent Shiite cleric, Grand Ayatollah Ali al-Sistani, urged people not to react illegally and maintain self-restraint and calm.

In Washington, a White House spokeswoman said the US condemned "such acts of senseless violence that are clearly aimed at undermining the Iraqi people's hopes for a peaceful and stable Iraq."

UK Foreign Secretary Margaret Beckett also condemned the "barbaric" attacks, saying that such actions "show how little the terrorists have to offer the Iraqi people and the importance of building national reconciliation".

The violence came a day after a UN report about Iraq that underscored the unbridled sectarian violence in Iraq. The report said that 3,709 civilians were killed in violence in Iraq in October -- the highest monthly toll since the war's start.