The blast on Sunday, which collapsed the roof of al-Hussein Mosque in the Musayyib area, also wounded 35 people.
"The collapse of the mosque roof killed many of those who were present," said Hamza Habib, who went to the scene after the blast.
"Blood was everywhere in the mosque, and I saw some body parts of victims."
Haidar, who was inside the mosque at the time of the explosion, said some people tried to stop the bomber, who was still able to detonate the explosives among the mourners.
The funeral was for a person who was killed two days before when a house in Musayyib was blown up, Nasser Karim said.
Sunday's bombing is just the latest in a series of sectarian attacks on mosques and funerals in central Iraq that have raised the specter of a return to the all-out Sunni-Shiite violence that peaked in 2006-2007 and killed tens of thousands of people.
The latest violence brings the death toll to almost 800 people in September and upwards of 4,600 this year, according to AFP figures based on security and medical sources.
HH/HH