The dead and wounded from the attack on the mosque in Iskandariya, 50km south of Baghdad, were taken to two hospitals, one in the town itself and another in Hilla, 95km south of Baghdad, senior doctors said.
Medical staff said there were eight killed and 13 wounded. The hospital in Hilla was also receiving casualties, a doctor said.
The blast at 7pm (1600 GMT) occurred as hundreds of people were gathered on the eve of Ashura, one of the holiest days in the Shia calendar.
The latest attack follows four blasts in Baghdad, which have killed at least 36 people and wounded dozens. The Baghdad bombings focused on Shias, who recently won the majority of seats in Iraq's first democratic elections in 50 years.
Survivors said a man wearing a "suicide belt" blew himself up on Friday in al-Kadhimain mosque in the Abu Dushair district south of Baghdad.
However, police sources said they thought rocket-propelled grenades had been fired.
Shias are marking the Islamic month of Muharram and Saturday is Ashura, the 10th day of Muharram and the holiest day of the year for them. The day marks the death of Imam al-Husain, the grandson of the Prophet Muhammad, in a 7th-century battle for leadership of the Islamic world.