Market bombings and other attacks across Iraq killed at least 40 people, and one senior intelligence figure said he could not rule out that guards may have taken bribes to allow terrorists to penetrate security during a Shiite pilgrimage.
The latest attacks added to fears that Iraq is descending further into violence after the last American troops withdrew late last year. More than 275 people have died in attacks over the past month, the bloodiest period since immediately after the US withdrawal.
Tuesday morning's wave of bombings struck six Iraqi cities and towns. The worst hit was Diwaniyah, 80 miles (130km) south of Baghdad, where an explosives-laden vegetable truck detonated in a crowded market. Officials said 26 people were killed and about 75 wounded.
A senior Iraqi military intelligence official said there were at least two security lapses in Tuesday's market attack, and money might have changed hands.
One guard at a security checkpoint in Diwaniyah failed to properly search the produce truck because he said he could not stand the smell of rotting vegetables and fruit, and another guard later allowed the truck to enter the market itself instead of being unloaded outside as security rules require, the intelligence official said. He said: "We do not rule out that bribes were paid to some at the checkpoints."
The attacks came as hundreds of thousands of Shiite Muslim pilgrims were heading to the holy city of Karbala this week for religious ceremonies set to peak on Friday. Shiite pilgrimages are a favourite target of Sunni insurgents linked to al Qaida. Attacks timed to strike during a similar march in Baghdad last month left 100 dead.
Diwaniyah is about 25 miles (40km) from Karbala, which was also hit by two bombs hidden in cars parked outside a market in the early morning, blasts that killed five people and wounded 30. Jubair al-Jabouri, chairman of the Qadisiyah provincial council, confirmed the death toll in Diwaniyah, a Shiite city and the provincial capital. He blamed al Qaida for the attacks. "Terrorism has no religion," Mr al-Jabouri said. "The terrorists targeted the innocents today in Karbala and Diwaniyah."
No one immediately claimed responsibility for the bombings.
Within hours of the two Karbala bombings, authorities banned vehicles from entering the holy city until Friday, a new step to protect the pilgrims. Karbala, 50 miles (80km) south of Baghdad, is the destination for annual Shiite rituals on the anniversary of the birth of the ninth-century Shiite leader known as the Hidden Imam.
Bombs also struck three other cities in central Iraq, and a gunman attacked security forces in a fourth. In the Sunni city of Taji, two bomb blasts killed three people and wounded 15. A policeman was among the dead, said security and health officials who confirmed the casualties. Taji is home to a military base and is 12 miles (20km) north of Baghdad. In the capital itself, two roadside bombs exploded next to security patrols in separate neighbourhoods, killing a policeman and a passer-by, and wounding 14 other people, officials said. And in Sunni-dominated Diyala province, just northeast of Baghdad, a bombing left two farmers dead, and a drive-by shooting killed two security officers and wounded two others.