Protesters threw stones at an armored personnel carrier and dozens of police in riot gear on a road outside the mosque. After protesters disregarded police calls to disperse peacefully, police fired the tear gas, and scattered the crowd which mostly fled back inside the mosque compound.
A voice on the mosque loudspeaker — where a small group of religious students appeared to be in control — appealed for the protesters not to attack security forces, but the situation remained tense.
The clashes spoiled a government attempt to reopen the mosque, which was stormed by the army July 10 after its pro-Taliban clerics had spearheaded a vigilante, Islamic anti-vice campaign that had challenged the government's writ in the Pakistani capital.
The military siege sparked a wave of violence by militants across Pakistan — aimed primarily at the country's security services — which coincided with a U.S. intelligence report labeling the country's lawless tribal region as a growing al Qaeda safe haven.
In the latest violence, gunmen opened fire Friday on a vehicle carrying the official spokesman for a provincial government in Pakistan's southwest, killing him, police said.
Raziq Bugti, spokesman and special adviser to the chief minister of Baluchistan province, died at the scene after unknown assailants fired a barrage of shots into his vehicle as it passed by a school in Quetta, said Javid Ahmed, a local police officer.
Earlier Friday, security forces stood by as protesters clambered onto the roof of the mosque and daubed red paint on the walls after forcing the retreat of a government-appointed cleric who was assigned to lead Friday prayers.
The mosque protesters demanded the return of the mosque's pro-Taliban former chief cleric, Abdul Aziz — who is currently in government detention — to lead Friday afternoon prayers, and shouted slogans against President Gen. Pervez Musharraf. Police fired tear-gas at the protesters after they refused to leave the mosque.
"Musharraf is a dog! He is worse than a dog! He should resign!" students shouted. Some lingered over the ruins of a neighboring seminary that was demolished by authorities this week. Militants had used the seminary to resist government forces involved in siege.
Despite questions over Musharraf's will, and ability, to tackle the apparent extremist revival gripping his country, the military ruler reiterated Friday his insistence that U.S. troops would not be allowed on Pakistani soil to help in the fight.
"It is very clear that here on Pakistani territory only Pakistani troops will operate. Nobody should have any doubt," Musharraf told journalists before leaving to visit Saudi Arabia and the United Arab Emirates, reports CBS News' Farhan Bokhari.
"It is the arrangement (with the U.S.), and we are capable of defending our area. We don't need any other force to help or assist us", Musharraf added.
President Bush said Saturday he was troubled by reports from the combined U.S. intelligence community that al Qaeda was regrouping and gaining strength in Pakistan's Northwest Frontier Province. Pakistani officials have since speculated that U.S. counterterror operations of some kind could be launched in the tribal region, flaunting the sovereignty of a crucial White House ally.
A senior Pakistani lawyer, who spoke to CBS News on condition of anonymity, said any U.S. incursion into Pakistani territory would likely break the relationship between Washington and Islamabad — and it may not be successful in netting the al Qaeda leaders thought to be hiding in the northwest region.
"Iraq is such an obvious case of U.S. intelligence failure. The U.S. went in on the pretext of finding weapons of mass destruction which have never been found. What is the evidence that U.S. intelligence on Pakistan is accurate?" he asked.
A senior Western diplomat in Islamabad, who also spoke to CBS News on the condition that he not be named, said the current trend in U.S.-Pakistan relations puts both countries in a difficult situation. "The U.S. is so deeply unpopular in the Islamic world that it needs allies, and Gen. Musharraf is among its few reliable allies."
The immediate crisis for Musharraf, however, is how to deal with the element of his population that is calling for a revolution, to oust him and impose fundamentalist Islamic law.
The crowd at the Red Mosque shouted support for the mosque's former deputy cleric, Abdul Rashid Ghazi, who led the siege against Musharraf's forces until he was shot dead after refusing to surrender.
"Ghazi your blood will lead to a revolution," the protesters chanted. Dozens clambered on the mosque's roof to continue the protest, including one waving a large black flag.
Armed police stood by on the street outside the mosque, but did not enter the courtyard where the demonstration was taking place.
In a speech at the main entrance to the mosque, Liaqat Baloch, deputy leader of a coalition of hardline religious parties, the Mutahida Majlis-e-Amal (MMA), condemned Musharraf as a "killer" and declared there would be an Islamic revolution in Pakistan.
"Maulana Abdul Aziz is still the prayer leader of the mosque. The blood of martyrs will bear fruit. This struggle will reach its destination of an Islamic revolution. Musharraf is a killer of the constitution. He's a killer of male and female students. The entire world will see him
hang," Baloch said.
Friday's reopening was meant to help cool anger over the siege, which triggered a flare-up in militant attacks and widespread anger that a religious site had been the scene of violence.
Pakistan's Geo television showed scenes of pandemonium inside the mosque, with dozens of young men in traditional Islamic clothing and prayers caps shouting angrily and punching the air with their hands.
Officials were pushed and shoved by men in the crowd. One man picked up shoes left outside the mosque door and hurled them at news crews recording the scene.
Maulana Ashfaq Ahmed, a senior cleric from another mosque in the city who was assigned by the government to lead Friday's prayers, was quickly escorted from the mosque, as protesters waved angry gestures at him.
Public skepticism still runs high over the government's accounting of how many people died in the mosque siege, with many still claiming a large number of children and religious students were among the dead. The government says the overwhelming majority were militants.
Mohammed Jesanjir, a 14-year-old student from a religious school elsewhere in Islamabad that is connected to the mosque, said Musharraf "will be humiliated like he humiliated the Quran," Islam's holy book, by ordering the mosque raid.
"God willing, we will take revenge," he said.
Wahajat Aziz, a government worker who was among the protesters, said officials were too hasty in reopening the mosque.
"They brought an imam that people had opposed in the past," he said. "This created tension in the environment. People's emotions have not cooled down yet."
Security was tightened in Islamabad ahead of the mosque's reopening, with extra police taking up posts around the city and airport-style metal detectors put in place at the mosque entrance used to screen worshippers for weapons.
Militants holed up in the mosque compound for a week before government troops launched their assault on July 10, leaving it pocked with bullet holes and damaged by explosions.
At least 102 people were killed in the violence. Attacks by militants in northwestern Pakistan along the border with Afghanistan have surged since the siege, killing about 200 others in suicide bombings and clashes.