ISLAMABAD, Pakistan - A suicide bomber set off explosives Friday in the midst of Shiite Muslims reciting the Quran, killing at least 20 and wounding dozens gathered for a religious festival at a shrine near the capital.

The motive was not immediately clear, but there are frequent sectarian attacks in this Islamic country by extremist elements of the Sunni and minority Shiite sects.

The schism between Sunnis and Shiites dates back to a 7th century dispute over who was the true heir to the Prophet Mohammed.

An AP photographer at the scene counted at least 20 bodies, many in pieces. An intelligence official said at least 20 people were killed and 150 were wounded.

The bomb ripped through hundreds of worshippers as they were reciting from the Muslim holy book beneath tents at the Bari Imam shrine outside Islamabad.

"It was like hell," said Syed Muktar Hussain Shah, 40, who had been waiting for a prominent Shiite leader, Hamid Moasvi, to address the gathering when the bomb went off. "I fell down ... when I woke up I saw dead bodies around me."

He and other witnesses said police collected the head of a suspected suicide bomber. Authorities did not immediately confirm that information.

The Shiite leader, Moasvi, was not hurt, witnesses said.

Mukhtar Kazmi, who was running a free clinic at the shrine, said it had treated about 200 people.

Police cordoned off the shrine and blocked access roads after the blast. The shrine is about half a mile from the official residence of Pakistan's prime minister, Shaukat Aziz.

Shiite worshippers, weeping and moaning, beat their chests in grief. Some clashed with police after officers charged the crowd with their batons to clear the way for ambulances.

Many also chanted, "Down with America!"