ISLAMABAD, Pakistan  —  A suspected homicide bomber struck Friday at a Muslim shrine close to the prime minister's official residence and the capital's diplomatic enclave, killing at least 20 Muslim worshippers.

After the blast, hundreds of Shiite pilgrims, beating their chests in mourning, clashed with baton-wielding police, who charged the crowd to clear the way for ambulances. Some of the demonstrators chanted, "Down with America!"

The explosion at the Bari Imam (search) shrine, burial place of an Islamic saint on the outskirts of Islamabad, was the latest attack on a religious gathering in Pakistan, which has a long history of violent sectarian rivalry. There was no immediate claim of responsibility.

President Gen. Pervez Musharraf (search) condemned the attack and appealed for Pakistanis to unite against "religious terrorism, sectarianism and extremism."

Security was stepped up in the capital, where the United States' top diplomat for South Asia, Christina Rocca (search), had met with Pakistani leaders on Thursday. The government instructed all four provinces to provide more protection for places of worship.

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Witnesses said the bomb went off in an open space near the shrine on the final day of a religious festival to honor the saint. Thousands of Sunnis and minority Shiites attend the festival each year.

The blast ripped through a congregation of hundreds of Shiites under a canvas tent put up to shade them from the sun. They were preparing for the arrival of Shiite leader Hamid Mosavi, a vehement critic of the U.S.-led war on terrorism, who was to deliver a sermon.

"There was announcement that Hamid Mosavi is coming. Everybody stood up and then there was the explosion," said Mohammed Ali, one of the worshippers. "Afterward, you couldn't identify anyone. Some had their legs blown off, some had their hands blown off. I lifted so many of the people and my clothes were soaked with blood."

Moasvi was not hurt, witnesses said.

Police cordoned off the shrine, which lies about half a mile from the official residence of Prime Minister Shaukat Aziz (search), and a similar distance from the diplomatic enclave where the U.S. and other embassies are located.

An Associated Press photographer at the shrine counted at least 20 bodies, many of them in pieces — scattered over about 150 feet — making it hard to give an exact figure. An intelligence official said at least 20 people were killed and 150 were wounded.

Information Minister Sheikh Rashid Ahmed said it appeared to be a homicide attack and at least 17 people died. He blamed "enemies of Pakistan and Islam."

Ahmed said authorities would release a photo of the suspected bomber, and announced a reward of $8,400 for help in identifying him.

Witnesses said police collected the head of a suspected homicide bomber — an unidentified man aged around 30 with a small beard and curly hair — from the blast scene. Authorities did not immediately confirm that information.

Ali Ahmad, an injured worshipper, said he had seen a man dressed in a police uniform who appeared to be the bomber walk inside the tent as worshippers recited the Koran. Police tried to stop the man but failed to prevent the attack, he said.

Another witness, S.M. Shirazi, gave a different account. He said two bearded men he thinks were the bombers entered the gathering and sat near a podium at the front. With the blast, he saw a body shoot through the roof of the tent.

Sunnis make up about 80 percent of Pakistan's 150 million people, and Shiites about 17 percent. Most live peacefully together, but extremist elements on both sides have a violent agenda. The schism dates back to a 7th century dispute over who was the true heir to the Prophet Mohammed.

In February, a gunbattle near the Bari Imam shrine left three dead. That violence was believed linked to a feud between two families over control of the shrine.