At least 95 people have been killed and 500 injured in six blasts near the government and diplomatic "Green Zone" in the centre of the Iraqi capital, Baghdad, the police said.
Witnesses said that the two main attacks on Wednesday appeared to target the foreign ministry and the finance ministry.
Police sources told Al Jazeera that the explosions, which took place within minutes of each other, were caused by lorries loaded with explosives which had been parked close to the buildings.
The blast outside the foreign ministry left a huge crater and severely damaged the building.
Major-General Qassim Atta, the spokesman for the Iraqi army's Baghdad operations, said: "A truck bomb went off near the Salhiyeh intersection and it caused casualties and a number of civilian cars were destroyed.
"We accuse the Baathist alliance of executing these terrorist operations," he said in an apparent reference to the political party of Saddam Hussein, the executed former president.
Television footage showed that the force of the explosions had blown out some of the windows of Iraq's parliamentary building.
Two mortars also landed inside the heavily protected "Green Zone", while a third landed outside.
The area, the site of government ministries and foreign embassies, has frequently been targeted with rocket and mortar fire.
Ahmed Rushdi, a journalist in Baghdad, told Al Jazeera: "These areas are supposed [to be] very secure ... it is not only checkpoints, you are always placing intelligence around this area to make it more secure.
"How are you going to say to people that Baghdad is now secure if you have so many explosions in this area?"
The attacks came six years to the day after a lorry bomb exploded outside the UN offices at the Canal Hotel killing 22 people.
Saad Muttalibi, an adviser to Iraq's ministry of national dialogue and reconciliation, said: "This is the continuation of the evil plans of people who cannot see a stable, free Iraq and people with the intention of keeping American forces in Iraq after the agreement that was signed for the Americans to leave.
"I think that this escalation of violence in Iraq is totally unacceptable as it is affecting the ordinary citizens," he told Al Jazeera.
Wednesday's attacks made it the bloodiest day in the Iraqi capital since June 24 when 62 people were killed after a bomb on a motorcycle rickshaw exploded in the predominantly Shia Muslim neighbourhood of Sadr City.
Larry Korb, a former US assistant secretary of defence who has advised US President Barack Obama on policy matters related to Iraq, said the attacks suggest that the Iraqi security forces are still not wholly loyal to the central government.
"They [the Iraqi security forces] are still riven by partisan and sectarian loyalties. There are 600,000 [security forces] - they should be able to provide the security that we had 150,000 Americans to do," he told Al Jazeera.
"It is really a question of whether [Iraqi leaders] have divided the power equitably and whether they have shared the economic resources in a way that people will not want to do [such attacks] in order to upset an order that they think excludes them."
Despite a reduction in violence in recent months, attacks on security forces and civilians remain common in Baghdad and the northern cities of Mosul and Kirkuk.
The number of violent deaths fell by a third last month to 275 from 437 in June, following the pullout of US combat forces from urban areas at the end of the month.