The explosion on Wednesday in the town of Howaydir was the latest in a wave of attacks against Iraq's Shia majority that many feel will push the country close to a full-scale communal conflict in the vacuum left by bickering politicians.
The car bomb in Howaydir exploded near a mosque and a crowded market.
Hospital officials said casualty tolls were expected to rise as ambulances were still rushing in with victims. Last week, a triple suicide bombing at a Shia mosque in Baghdad killed up to 90 people.
American military officials say Abu Musab al-Zarqawi, the al-Qaida chief in Iraq, has shifted his tactics, attacking Iraqi army and security forces instead of US troops while continuing his bombings against civilians.
US military deaths in the Iraq war declined for a fifth straight month in March while insurgent attacks, aimed increasingly at Iraqis, continued unabated.
But rebels have killed eight US soldiers since Sunday in a series of incidents, including one struck by a roadside bomb on Wednesday.
Also during the day, a policeman and three civilians were killed and four people wounded when a roadside bomb struck a police patrol car in central Baghdad, police said.