At least 58 people have been killed in a suicide bomb attack in the town of Musayyib, some 60 km (40 miles) south of Baghdad.
Police told the BBC the bomber blew himself up near a mosque. The blast caused a nearby fuel tanker to explode.
At least another 80 people are said to have been injured.
The blast, the worst single attack in over two months, follows a week of violence in which 100 were killed in 16 suicide attacks in Baghdad.
On Friday alone, 10 suicide bombers blew themselves up in a wave of attacks across the city.
In a statement posted on the internet afterwards, the militant group al-Qaeda in Iraq said their leader Abu Musab al-Zarqawi had urged them to intensify their attacks.
Earlier on Saturday, three British soldiers were killed by a roadside bomb in central Amarah, south-east Iraq.
Musayyib is a mixed town near Kerbala where the majority of the population is Shia.
MAJOR RECENT ATTACKS
15 July: Suicide bombs kill 16
13 July: Bomb kills 26 children
10 July: 20 army recruits killed
26 June: 35 die in Mosul attack
25 June: Suicide attacks kill 23
20 June: Several attacks, 31 dead
2 June: Multiple bombs kill 24
30 May: 27 dead in Hilla
11 May: 70 dead in Tikrit, Hawija
4 May: Irbil bombing kills 60
There have been bombings there in the past, but nothing on this scale, says the BBC's Richard Galpin in Baghdad.
"This is a black day in the history of the town," Musayyib police chief Yas Khudayr told Reuters news agency.
The attacker in Musayyib is thought to have detonated explosives that blew up a petrol tanker near the Shia mosque and a market.
The explosion set neighbouring houses on fire and caused very serious damage, an interior ministry official told the AFP news agency.
The police say there are so many casualties that ambulances are having to take them to hospitals in several neighbouring towns.