At least 50 people have been killed and dozens injured in apparent suicide attack outside an Iraqi police station, hospital officials say.
Most of those killed were Iraqis
The truck bomb exploded in the mainly Shia Muslim town of Iskandariya, 40km (25 miles) south of Baghdad, as new recruits were queuing outside.
US troops cordoned off the area to hold back crowds chanting anti-US slogans.
The attack is the latest in a series targeting Iraqis working with the American-led forces in the country.
It comes a day after US officials warned against what they described as an al-Qaeda plot to ignite violence between the majority Shia Muslims and the Sunni Muslims who held power under the former Iraqi regime.
The BBC's Stephen Sackur in Iskandariya reports that tension is extremely high in the town and US troops are struggling to maintain order.
Abdulrazak Al-Jelabi, the manager of the local hospital, told the BBC that he had counted 50 bodies.
Iraqi police said they suspected a suicide attacker had delivered the bomb in a pick-up truck. They said the truck used to belong to a former Iraqi intelligence officer in Saddam Hussein's regime.
Brig Gen Mark Kimmitt, deputy operations chief of coalition forces in Iraq, said it was too early to say who was behind the attack, and that it was unclear whether it was a suicide attack.
But he said the blast had "some of the fingerprints" of al-Qaeda, the terror network blamed for the 11 September 2001 attacks in the US.
On Monday, Brig Gen Kimmitt had revealed that documents seized in Baghdad showed that al-Qaeda planned to target Shia Muslims in order to fan sectarian conflict.
"There is clearly a plan on the part of the outsiders to spark civil war, commit sectarian violence, try to expose fissures in society," he said.
Tuesday bombing is the worst loss of life in Iraq since twin bomb attacks in the northern city of Irbil on 1 February killed more than 100 mainly Kurdish people during holiday celebrations.
RECENT MAJOR ATTACKS
10 February: At least 35 killed in car bomb attack on police station in Iskandariya
1 February: At least 100 killed by twin suicide bombings during celebrations in Kurdish city of Irbil
18 January: 18 reported killed outside coalition HQ, Baghdad
14 December: Car bomb at police station kills 17 in Khalidiya, west of Baghdad
12 November: 26 die in suicide attack on Italian base in Nasiriya
The latest bomb exploded at 0930 (0630 GMT), said local police commander Lt Col Salam Trad.
Coalition troops sealed off the town and used helicopters to ferry some of the wounded to a Polish field hospital nearby.
A photographer for AFP news agency saw a crater approximately 25 metres (yards) wide outside the police station which stands on the main road to Baghdad.
The facade of the one-storey building was damaged and the blast had wrecked about 15 cars.
At the hospital, staff tried desperately to mop up pools of blood with soap and water.
Coalition forces reported no casualties in the blast.