It was the second attack in three days targeting visitors to the al-Askari mosque in the former insurgent stronghold of Samarra, north of Baghdad, for commemorations of the death of a ninth century religious figure who is buried there.
Witness Kamil Mamdoh described mayhem, with police shooting into the air to disperse the hysterical crowd.
"After we heard a thunderous blast, we rushed to the scene," Mamdoh said. "The ambulances were filled with casualties, and some of the wounded wore torn clothes covered with blood."
The shrine is still being rebuilt after its golden dome was destroyed in a Feb. 22, 2006, bombing that was blamed on al-Qaida in Iraq and sparked years of retaliatory bloodshed between Sunnis and Shiites. Officials said the mosque was not damaged in Saturday's attack.
A Samarra policeman said the bomber joined the busload of pilgrims in a parking lot about two miles (four kilometers) from the mosque and detonated his explosives-packed vest. The blast set fire to at least eight cars and buses parked in the lot, he said.
Samarra is 60 miles (95 kilometers) north of Baghdad.
Police and hospital officials in Samarra and the nearby city of Balad confirmed the death toll and said about 30 other people had been wounded.
All officials spoke on condition of anonymity because they were not authorized to release the information.
The destruction of the Shiite mosque in 2006 helped send Iraq into a downward spiral of violence that left whole neighborhoods around the country divided by sect. The day after that attack, nearly 140 people were killed, and tens of thousands died over the next two years.
In June 2007, another bombing brought down the twin minarets on the mosque's compound.
The shattered shrine remained vacant during most of the turmoil, but the Shiite-dominated government made it a priority to rebuild it and bring the pilgrims back as part of national reconciliation efforts.
As recently as last week, U.S. officials were hoping to remove a half-mile (800 meters) passageway of blast walls leading up to the mosque, starting later this month in a symbolic if risky demonstration of Samarra's security gains over the last few years.
But Sunday's attack, combined with a Thursday car bombing that killed eight pilgrims and wounded more than 40 who were on their way to Samarra, showed how vulnerable the area remains. Pilgrims are an easy target for Sunni insurgents who plant bombs along highways where large groups walk on the way to shrines.
Associated Press Lara Jakes contributed to this report.