The suicide bomber detonated an explosives-rigged minibus during morning rush hour at a checkpoint at the northern entrance to Hilla, the confessionally mixed but mostly Shiite capital of Babil province south of Baghdad.
The attack killed 37 people and wounded 166, a police lieutenant and a doctor at Hilla hospital said, speaking on condition of anonymity. Among the fatalities were five policemen, two women and five children, they said.
"I saw a huge fire that covered the entire checkpoint and many cars nearby," Salam Ali, who suffered wounds to his chest and a hand, said from his Hilla hospital bed.
"Many victims could not get out of their cars because the pressure of the explosion fused the doors shut."
Another witness, 18-year-old Kadhim Abdulhussein, said he saw pieces of metal from the checkpoint scattered dozens of metres (yards) from the scene of the attack.
Iraqiya state television said two of its employees, Muthanna Abdulhussein and Khaled Abed Thamer, were among the dead.
Militants carry out frequent attacks on security forces, and also target areas where crowds gather. The checkpoint combined the two.
Just north of Hilla, a gun attack on a police checkpoint left two policemen dead and four others wounded.
In Abu Ghraib, west of Baghdad, gunmen shot dead at least two soldiers and wounded one at an army checkpoint, while six attacks north of the capital killed three policemen and two soldiers and wounded nearly 40.
- PM blames Saudi, Qatar -
In an interview broadcast on Saturday, Maliki accused Saudi Arabia and Qatar of backing militant groups in Iraq, saying they have effectively declared war on the country.
The two Sunni Gulf states "are attacking Iraq, through Syria and in a direct way, and they announced war on Iraq, as they announced it on Syria, and unfortunately it is on a sectarian and political basis," the premier told France 24 television.
"These two countries are primarily responsible for the sectarian and terrorist and security crisis of Iraq."
Saudi Arabia and Qatar have emerged as regional rivals.
The two countries support fighters opposed to embattled Syrian President Bashar al-Assad, and in recent weeks they have sparred over Doha's backing for the Muslim Brotherhood of deposed Egyptian president Mohamed Morsi.
Baghdad has long complained support for militant groups fighting in Syria's civil war finds its way through to Iraq, with weapons in particular ending up in jihadist hands.
In the interview, Maliki said Riyadh and Doha provide political, financial and media support to militant groups, and accused them of "buying weapons for the benefit of these terrorist organisations".
He also accused Saudi Arabia of supporting global "terrorism".
Maliki condemned "the dangerous Saudi stance" of supporting "terrorism in the world -- it supports it in Syria and Iraq and Lebanon and Egypt and Libya, and even in countries outside" the Arab world.
Violence in Iraq has reached a level not seen since 2008, when the country was just emerging from a brutal period of sectarian bloodshed in which tens of thousands of people died.
More than 150 people have been killed so far this month and upwards of 1,850 since the beginning of the year, according to AFP figures based on security and medical sources.