A leading human rights group called for an independent investigation Wednesday into the Pakistan military's air raid that killed 80 people at a seminary the government claimed was used for militant training and had been visited by Al Qaeda's No. 2 leader.

Pakistan's top army spokesman said the military had no choice but to launch a helicopter-fired missile barrage to destroy the building and kill all inside, rather than try to arrest the suspected militants and risk them escaping.

Monday's attack on the religious school in the Bajur tribal district village of Chingai has sparked huge protests and a threat by a pro-Taliban elder to send waves of suicide bombers against security forces.

On Wednesday, 10,000 tribesmen, many brandishing firearms, rallied in Salazi village, near the Bajur tribal region's main town of Khar where the attack happened, railing against President Gen. Pervez Musharraf and U.S. President George W. Bush and calling for their deaths. The mass rally was the third held in the area in the three days since Monday's attack. Scores of students also protested in the northwestern city of Peshawar.

In a statement, New York-based Human Rights Watch urged Pakistan's government to allow independent investigators to visit the area to determine who carried out the attack, how it was planned and executed, and who was killed.

"The onus is on the Pakistani government to provide a credible account of the legitimacy of the attack resulting in the deaths of so many," the group's South Asia researcher Ali Dayan Hasan said, adding the high number of dead pointed to use of excessive force.

Pakistan's military said the school — known as a madrassa — was destroyed by missile-firing helicopters because it was preparing dozens of students to launch attacks in this South Asian nation and neighboring Afghanistan.

Chief army spokesman Maj. Gen. Shaukat Sultan said Pakistani forces had no option but to bomb the madrassa, saying any operation by ground forces to surround and arrest the suspects inside the building could have tipped them off and led to their escape.

"The biggest factor that contributes to success is surprise," Sultan told The Associated Press. "It was important to take them."

Evidence for the attack included students aged in their 20s being seen conducting physical exercises; madrassa leaders, including Al Qaeda linked Hussain, telling public rallies that they were preparing suicide bombers, and other intelligence he declined to specify, Sultan said.

Pakistani troops have been preventing journalists, human rights monitors and political leaders from traveling to the site of the attack, preventing independent observers from getting a firsthand look at the scene.

At a security seminar Tuesday in Islamabad, Musharraf said the targeted compound was being monitored for the last several days and that militants had been training there.

"We knew what they were doing," he said. "There were no innocent people inside the compound and anyone saying otherwise is telling lies ... Those killed were militants using weapons."

A security official said Al Qaeda's No. 2, Ayman al-Zawahiri, and the alleged mastermind of the London airline bomb plot busted in August, Egyptian Al Qaeda operative Abu Ubaida, had both visited the religious school several times, but they weren't there at the time of Monday's raid.

At Tuesday's huge rally in the Bajur town of Khar, near Chingai, local pro-Taliban elder Inayatur Rahman told the crowd he had prepared a "squad of suicide bombers" to target Pakistani security forces in the same way militants are attacking in Afghanistan and Iraq.

The protests come at a volatile time for Musharraf, who has been under U.S. and Afghan pressure to crack down on militants operating along the Pakistan-Afghan frontier where Al Qaeda and Taliban militants are believed to roam.

Locals claimed unmanned U.S. Predator drones were flying above the village before the missile strikes. Pakistan and U.S. military officials denied any American involvement.

The attack has also raised questions about Pakistan's strategy against militancy along the Afghan border, and whether it favors dialogue or military force.

In September, the government reached a controversial peace deal with pro-Taliban tribesmen after months of bloody battles in another restive tribal region, North Waziristan. A similar deal was reportedly in the offing in the less volatile Bajur region before Monday's attack.

Ali Mohammed Jan Aurakzai, governor of North West Frontier Province, acknowledged to reporters on Wednesday the attack could delay an agreement with tribesmen in Bajur, but maintained the peace process would continue.

In January, a U.S. Predator drone fired a missile targeting al-Zawahiri in Damadola, near Chingai, missing the Al Qaeda No. 2 but killing 13 civilians and sparking anti-U.S. protests across Pakistan. Pakistan claimed several senior Al Qaeda figures were also killed in the missile attack — including Ubaida — but their bodies were never found.