A large explosion near the town of Mahmudiya, south of Baghdad, has killed at least 18 people and hurt about 28.
The explosion left a deep crater in the middle of a busy street
Police said it had been caused by a car bomb and it was targeting supporters of radical Shia cleric Moqtada al-Sadr.
He has urged protesters converging on the Shia holy city of Najaf to hold mass protests on Monday - the fourth anniversary of the fall of Baghdad.
They will call for the withdrawal of coalition troops. Six US soldiers were killed in separate attacks on Sunday.
Three were killed by a roadside bomb during a patrol south of Baghdad, the military said in a statement. Another died and three were wounded by a mortar or rocket attack in a separate incident south of the capital.
Two more soldiers died from wounds sustained in combat in the Diyala and Salahuddin provinces, north of Baghdad.
A 24-hour ban on movement by all vehicles, for fear of car bomb attacks, has now been called in the capital from 0500 (0100 GMT) on Monday.
Mahmudiya, about 30km (20 miles) from Baghdad, has seen frequent bombings and attacks in recent years.
Police sources said the blast there struck a residential building in a busy central area. Earlier reports had said it was a rocket attack.
The force of the blast reduced most of the building to rubble and there are fears that the final death toll could rise as more bodies are discovered.
"Three of us and a young boy were sitting in a store selling spare parts for cars when there was a huge explosion. Debris from the roof fell on me," one survivor, Sadeq, told the Reuters news agency.
Mahmudiya, a mixed Sunni-Shia town, is in a tense and volatile area that has seen frequent sectarian killings.
It lies in a belt of territory to the south of Baghdad which has seen a hotbed of insurgent attacks as well as friction between Sunni and Shia communities.
Elsewhere, police said a car bomb had killed five people and injured six in Ilam, in south-western Baghdad.
Also on Sunday, the US military said four of its troops had been killed in an explosion near their vehicle in Diyala province, north of Baghdad, another area hit by frequent violence.
Further south, in the mainly Shia city of Diwaniya, hundreds of US and Iraqi army troops are battling for a third day with Shia militiamen holding positions in the centre of the city.
Diwaniya has been under curfew since the operation began at dawn on Friday and US air strikes were called in to try to root out the militia fighters.
Up to 30,000 new US troops have been assigned to Iraq for a security crackdown on militia groups, which began two months ago.