More than 60 people have been killed and 131 injured in three car bombs in Shia districts of the Iraqi capital Baghdad, police sources have said.
New Baghdad has been frequently hit in the past
Twin blasts hit a market in the New Baghdad area, killing 60 people. Two more people died in another blast in Sadr City.
The attacks are the deadliest since a joint US-Iraqi security offensive was launched on Wednesday.
Earlier, Iraq started reopening border crossings with Iran and Syria.
The borders had been closed for three days as part of the crackdown, but two main routes into Syria and four into Iran have now re-opened, a senior security official in Baghdad said. Other routes will be opened gradually.
In the capital, a large plume of smoke could be seen rising over New Baghdad, an area attacked frequently in the past, following two blasts at the open-air market.
One hit vegetable stalls, and a second shortly afterwards detonated near a row of electrical goods shops, AFP news agency said.
"Where is the security plan?" shouted grieving relatives outside a hospital.
Shortly afterwards, a car packed with explosives rammed a police checkpoint in Sadr City, killing two people and injuring 10.
It has also been revealed that two US soldiers were killed while on patrol in Baghdad on Saturday. One was shot and the other died when a grenade was thrown at his vehicle.
The blasts came a day after US Secretary of State Condoleezza Rice visited Baghdad and said the security operation had got off to a good start.
On Friday, Iraqi Prime Minister Nouri Maliki described the crackdown - which has seen thousands of extra US and Iraqi troops sent to Baghdad - as a "brilliant success".
The BBC's Jane Peel in Baghdad says the latest attacks would seem to confirm the opinion of senior US military officials that the lull in violence in recent days was temporary and that there were difficult days ahead.
Sadr 'not in Iran'
The temporary closure of the borders came amid claims by the US that Iran is supplying militias with sophisticated weapons to attack US troops. Tehran rejects the accusation.
There have been reports in recent days that radical Iraqi Shia cleric Moqtada Sadr and supporters from his Mehdi Army had fled to Iran ahead of the crackdown.
British troops were involved in securing the border with Iran
But a spokesman for Iran's foreign ministry, Mohammed Ali Hosseini, denied the claims.
"Once the Americans claimed he fled from Iraq, two weeks later they claimed he wanted to enter Iran, but the Iranians didn't give him a visa," Mr Hosseini said.
"They have been fabricating such false claims. These baseless claims are a psychological war driven by America in Iraq in order to put more pressure on Iran. No, he is not in Iran."
Meanwhile, the leaders of Iran and Syria have accused the US, which has 140,000 troops in Iraq, of meddling in that country and in Lebanon to divide the region's Muslims.
"They want to push the peoples and the governments to make use of ethnicities and create divisions in the Islamic world. It is this final card that they are trying to play," Syria's President Bashar al-Assad said before leaving Tehran after a two-day visit.