Islamist political parties have added their voice to charges that Pakistani President Pervez Musharraf's bid for re-election is unconstitutional. The movement against Mr. Musharraf's candidacy was also joined Thursday by terrorist leader Osama bin Laden, who called for a holy war against the president. Daniel Schearf reports from Islamabad.

Hundreds of protesters, mostly from Islamist political parties, chanted slogans Friday outside Pakistan's Supreme Court building demanding Mr. Musharraf's resignation.

Mr. Musharraf, who came to power in a 1999 military coup, is seeking re-election as president in October. But his candidacy is being challenged in the Supreme Court by opposition parties, who say it violates the constitution.

Critics say Mr. Musharraf should not be allowed to be both president and leader of the military. The religious parties are also opposed to his cooperation with the United States in the fight against terrorism.

Pakistani officials and Islamist politicians had close ties with the Taleban rulers of Afghanistan before the U.S. was attacked by al-Qaida on September 11, 2001.

After those attacks, Mr. Musharraf's government led a crackdown on supporters of al-Qaida and the Taleban, and has since received billions of dollars in aid from the United States. This angers Islamists, who see the U.S. "war on terror" as a war against Islam.

In a new audio message released on the Internet Thursday, al-Qaida leader Osama bin Laden urged Pakistanis to overthrow Mr. Musharraf's government.

Bin Laden called Mr. Musharraf an "infidel," or non-believer, for launching a raid last July on a mosque in Islamabad used as a base by religious extremists. More than 100 militants were killed in the raid.

The presidential spokesman, Major General Wahid Arshad, brushed off the bin Laden threat. He says Pakistan is fighting extremism for its own interests, not for the U.S. or anyone else.

"Well, as far as the military is concerned, I think we don't give much importance to such threats," he said. "We've been hearing such threats before and it doesn't really deter us from our aims and objectives, which we have as part of our national effort to eradicate extremism and terrorism."

Opposition politicians have threatened to resign from parliament and hold nationwide demonstrations if Mr. Musharraf is allowed to run for president while chief of the Army.

He has vowed to give up his military position, but only after the elections and only if he wins.