A wave of car bombs and mortars in Baghdad's Sadr City district has left at least 144 people dead and more than 200 injured, Iraq police have said.
Several blasts struck in quick succession
It was the most devastating series of attacks in Iraq's capital in a single day since the US-led invasion in 2003.
The bombings in Sadr City were followed by mortar attacks on Sunni areas.
The Iraqi authorities have put Baghdad under curfew and closed the airport, and Iraqi PM Nouri Maliki went on TV to appeal for restraint amid the violence.
"We denounce sectarian practices that aim to destroy the unity of the nation," Mr Maliki said.
Leaders of Iraq's Shia, Sunni and Kurdish communities also appealed for calm, while Iraq's most prominent Shia cleric, Grand Ayatollah Ali al-Sistani, "urged people not to react illegally and maintain self-restraint and calm," one of his officials said.
In Washington, a White House spokeswoman said they 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 blasts brought panic to the streets of Sadr City, a densely populated, largely Shia neighbourhood, as distraught residents searched for family and friends.
Several car bombs, at least three of them believed to be suicide attacks, exploded minutes apart.
Among the targets were a busy square, a food market and a street where people catch buses.
"I was out shopping. As the bombs went off, everyone started running and shouting," news photographer Kareem al-Rubaie told Reuters new agency.
"I saw a car from a wedding party, covered in ribbons and flowers. It was burning. There were pools of blood on the street and children dead on the ground."
People tried to pull bodies out of the mangled wreckage of cars and minibuses and put out fires.
Several mortar rounds were also fired into hit Sadr City, police said.
The number of casualties put major pressure on transport and hospitals.
BLOODIEST DAYS OF VIOLENCE
23 Nov 2006 - 144 dead
Wave of car bomb and mortar blasts strike Sadr City in Baghdad
7 April 2006 - 85 dead
Triple suicide bombing at Shia Buratha mosque in Baghdad
5 Jan 2006 - 110 dead
Suicide bombers hit Karbala shrine and police recruiting station in Ramadi
14 Sept 2005 - 182 dead
Suicide car bomber targets Baghdad labourers in worst of a series of bombs
28 Feb 2005 - 114 dead
Suicide car bomb hits government jobseekers in Hilla
2 March 2004 - 140 dead
Suicide bombers attack Shia festival-goers in Karbala and Baghdad
1 Feb 2004 - 105 dead
Twin attacks on Kurdish parties' offices in Irbil
The injured filled Sadr City's hospitals, with dozens lying bleeding in the corridors.
Angry residents and armed Shia militiamen came out onto the streets, shouting curses at Sunni Muslims, the Associated Press news agency reports.
The Iraqi health minister, Ali al-Shammari, accused Sunnis and loyalists of the former ruling Baath party of being behind the attacks.
"They were killed in cold blood by Sunni extremists and Baathist criminal remnants," he told the BBC Arabic Service.
The bombs exploded shortly after dozens of gunmen had attacked the health ministry in Baghdad, clashing with Iraqi guards and soldiers before they were beaten off.
Sadr City is largely controlled by the Mehdi Army, the best-known of the Shia Iraqi militias, which has been accused of carrying out many sectarian attacks.
Shortly after the blasts, a dozen mortars hit the Adhamiya district, a predominantly Sunni area, injuring 10 people, the interior ministry said.
The Iraqi authorities put Baghdad's seven million residents under curfew on Thursday evening, saying all people and vehicles must stay off the streets until further notice.
The Iraqi authorities have also closed Basra's air and sea ports in the south, as well as the international airport in Baghdad.
The daily attacks in Baghdad are now more brazen and more sectarian, says BBC Middle East analyst Roger Hardy.
On Wednesday, the United Nations said violent deaths among civilians hit a record high in October, with more than 3,700 people losing their lives - the majority in sectarian attacks.