Athousand young men storm Islamabad’s Aabpara Junction, shearing through waves of tear gas. The police retreat, attempt to hold some kind of line, then flee in disarray. On reaching the junction, three older men unfurl a banner praising THE HONOR OF THE PROPHET. The protesters had vowed they would hold their demonstration. And–despite a police ban, mass arrests and a massive security clampdown on Pakistan’s capital–they did.

The protest was ostensibly about the cartoons of the Prophet Muhammad published in several European newspapers. But it was also the latest shot in what some Pakistani analysts are calling the most serious challenge to President-General Pervez Musharraf since he seized power in a coup in 1999. The challenger is the Muttahidda Majlis-e-Amal (MMA), a parliamentary coalition of Pakistan’s main Islamist parties and once an ally of Musharraf’s military regime. What brought about the rupture?

Pakistani protests over the cartoons had been small, peaceful and confined to the Islamists. They are now large and savagely violent. In Lahore and Peshawar, where 60,000 participated, shops were looted, government buildings attacked and police fired upon, leaving at least five dead, all civilians. The rioters were not just the bearded madrasa students of Taliban vintage; they were the young urban poor, disenfranchised by economies of conspicuous consumption that have left them beached. But the riots were not the spontaneous revolt of an underclass. They were instigated by Pakistan’s oldest and most powerful Islamist party, the Jamaat Islami (JI), which has forged a potent alliance between its devout middle-class cadre and alienated youth who have “found sanctuary in its Islamic rhetoric and institutions,” says analyst Hasan Askari Rizvi.

The JI is the dominant faction in the MMA. It has long had close relations with the Pakistani military, including its powerful Inter-Services Intelligence agency, which has deep jihadi roots: The ISI funded and trained not only Afghanistan’s anti-Soviet mujahedeen but also the Taliban and the Islamist insurgency in Kashmir. The JI was an integral part of the policy, its jihadist arms serving the ISI’s foreign wars in Afghanistan and Kashmir.

Since 9/11, when Musharraf changed sides and joined Washington in the “war on terror” and the MMA was formed, the JI has made an uneasy pact with the regime: conniving with Musharraf’s constitutional ruses to extend his one-man rule in return for a deeper stake in Pakistan’s political system. Today the MMA holds power in Pakistan’s North West Frontier Province, shares power in another and is the largest opposition bloc in the National Assembly. Politically, the Islamists in Pakistan have never been so powerful.

But the pact has started to fray. In the past six months the JI has assailed Musharraf’s botched response to the Pakistani earthquake; his collusion with US forces in tracking down and killing Taliban and Al Qaeda suspects on Pakistan’s border with Afghanistan; and his plans for new legislation curbing Pakistan’s madrasa system. The JI has seized on the furor caused by the cartoons to turn discontent into confrontation. “The days of the military government are numbered,” says JI leader Qazi Husain Ahmad. “We will not allow President-General Musharraf to remain in power.”

But Ahmad knows that neither the JI nor the MMA has the support or the firepower to bring down the Musharraf regime. What the Islamists can do is foment “the conditions so that Musharraf either accepts their demands or the military is compelled to move against him,” says Pakistani commentator Zaffar Abbas.

The JI was the driving force behind protests that ousted Pakistan’s first military ruler, Ayub Khan, in the 1960s and the government of Zulfikar Ali Bhutto in the 1970s. It was the principal opposition to Benazir Bhutto’s and Nawaz Sharif’s governments in the 1990s. All those struggles were invoked in the name of Islam against the secularism, decadence and pro-Western agenda of the leader. All, too, were supported by the army and the ISI.

There is no evidence of such complicity this time around. The aim, rather, is to again make that alliance possible, says Abbas. “The JI is convinced that Musharraf is the only thing that stands between them and the military establishment–an establishment they believe shares their vision of Islam for Pakistan. They are gunning for Musharraf and for him alone.”

How will he respond? The MMA has planned a rolling campaign of protests to climax with George W. Bush’s visit to Pakistan in March. If Musharraf moves to ban them, he risks the debacle of Islamabad. If he doesn’t, he risks the violence of Lahore and Peshawar. Whatever he does, there is a growing sense that The Leader is losing control. “And it is when Musharraf loses control that the army will move against him,” says former general and now antinuclear campaigner Talat Masood. “The Pakistani military doesn’t back losers.”