Up to 60 people have been killed in a series of car and suicide bombings mainly in Shia areas in and around Iraq's capital, Baghdad, officials say.
The co-ordinated attacks targeted markets, restaurants, bus stops and day labourers during the morning rush hour.
Iraq's deadliest day in six months came on the 10th anniversary of the US-led invasion that toppled Saddam Hussein.
Violence has decreased in Iraq since the peak of the insurgency in 2006 and 2007, but bombings are still common.
Sunni Islamist militants linked to al-Qaeda have vowed to step up attacks on Shia targets and state officials this year in an attempt to provoke sectarian conflict and weaken the Shia-led government.
In a sign of concern over the security situation, the cabinet announced that it was delaying elections scheduled for 20 April in the provinces of Anbar and Nineveh by up to six months.
Police sources told the BBC that more than 150 people were also injured in Tuesday's violence, which are reported to have included at least 15 car bombings as well as several roadside bombings and shootings.
Most of the attacks took place in predominantly Shia districts of Baghdad over a period of about two hours during the morning rush hour.
The first occurred at around 08:00 (05:00 GMT), when a bomb exploded outside a restaurant in the eastern district of Mashtal, killing four people and damaging several cars, according to the Associated Press.
Minutes later, two day labourers were killed by a roadside bomb planted where they gather every day hoping to pick up work in New Baghdad, a neighbouring area just to the south.
In the north-eastern district of Sadr City, five people died when a bomb was detonated beside a police patrol, and three commuters were killed by a device stuck to the underside of a minibus in which they were travelling, AP reported.
Another two people were killed by a blast on a commercial street in the area.
"I was driving my taxi and suddenly I felt my car rocked. Smoke was all around. I saw two bodies on the ground," Ali Radi, a taxi driver who was in Sadr City at the time of one of the attacks, told the Reuters news agency.
"People were running and shouting everywhere."
The deadliest of Tuesday's attacks appeared to have taken place near the Ministry of Labour and Social Affairs in the eastern district of Qahira. Seven people were killed there and another 21 wounded, officials said.
Another six people died and 15 were hurt when a bomb exploded outside a restaurant close to a main entrance to the heavily-fortified Green Zone, which houses government offices and the embassies of several countries. A plume of black smoke was seen over the capital afterwards.
Attacks were also reported in the Shia districts of Husseiniya, Zafaraniya, Shula and Utafiya, as well as the Sunni district of Tarmiya.
A mortar shell landed near a clinic in the town of Taji, north of Baghdad, killing two people, while a suicide bomber targeted a bus stop in Iskandariya, south of the capital, killing five people, AP said.
Police also said three improvised explosive devices (IEDs) had been set off and weapons fired in the northern province of Kirkuk.
The attacks came amid heightened security in Baghdad, which saw new checkpoints set up and key roads closed, the AFP news agency reported. Soldiers and police were searching government vehicles which would usually be allowed to pass uninspected, it added.
Tuesday is believed to have been the deadliest day in Iraq since 9 September last year, when 76 people were killed in a wave of attacks, some of which targeted the security forces.
The BBC's Jim Muir in Baghdad says no group has said it was behind Tuesday's violence, but the Islamic State of Iraq, a militant umbrella group that includes al-Qaeda in Iraq, has in recent months stepped up its attempts to revive the insurgency.
Such co-ordinated attacks are not unusual, but it is assumed that the timing may have been chosen to coincide with the 10th anniversary of the invasion, our correspondent adds.
It comes at a time of deep political crisis in the country, with the Prime Minister Nouri Maliki sharply at odds with a wide range of political forces, including the Kurds, most of the Sunni groups, and many factions within his own Shia community.