Pakistani troops backed by missile-firing helicopters on Monday struck a religious school purportedly being used as an Al Qaeda training center, killing 80 people in what appeared to be the country's deadliest-ever attack against suspected militants.

The country's top Islamic political leader said American planes were used in the pre-dawn strike against the school -- known as a madrassa -- and called for nationwide protests Tuesday, claiming all those killed were innocent students and teachers. A U.S. military official denied any involvement in the attack in northwestern Pakistan, near the Afghan border.

An Al Qaeda-linked militant who apparently was a primary target of the strike had left the building a half hour beforehand, a Pakistani official said.

Anger over the missile strike scuppered the signing of a peace accord, expected Monday, between tribal elders linked to militants. The United States has urged Pakistani President Gen. Pervez Musharraf to do more to stop militants from crossing from tribal regions into Afghanistan, where Taliban-fanned violence has reached its deadliest proportions since the American-led invasion in 2001.

Musharraf, along with Afghan President Hamid Karzai, met with President Bush last month in Washington to address the issue.

Helicopter gunships fired four to five missiles into the madrassa in Chingai, said army spokesman Maj. Gen. Shaukat Sultan. The blasts tore apart the building and all inside, spraying body parts, blood and debris across a wide area.

Sultan said initial estimates indicate the attack killed about 80 suspected militants from Pakistan and other countries. Only three people -- all seriously wounded -- were believed to have survived, a hospital official said.

"These militants were involved in actions inside Pakistan and probably in Afghanistan," Sultan told The Associated Press.

Sultan said the attack was launched after those in charge of the building refused warnings to close it down.

Among those killed was Liaquat Hussain, a Pakistani cleric and associate of Al Qaeda No. 2 Ayman al-Zawahiri, locals and an intelligence official said. Another al-Zawahiri deputy, Faqir Mohammed, was believed to have been in the madrassa and left 30 minutes before the strike, according to the official, who spoke on condition of anonymity as he was unauthorized to speak to the media.

In Islamabad, Qazi Hussain Ahmed -- an opposition political leader -- blamed the U.S. for the attack and said claims that the madrassa was a terrorist training center were "rubbish." Thirty children were among the dead, he claimed.

"It was an American plane behind the attack and Pakistan is taking responsibility because they know there would be a civil war if the American responsibility was known," said Ahmed.

In Afghanistan, U.S. military spokesman Maj. Matt Hackathorn denied the U.S. was involved in the strike.

"It was completely done by the Pakistani military," he told the AP.

Mohammed -- the Al Qaeda deputy who escaped the raid -- addressed a crowd of 10,000 mourners at a mass funeral for the victims, criticizing Musharraf's government and promising widescale protests.

"We were peaceful, but the government attacked and killed our innocent people on orders from America," Mohammed told the rally as dozens of militants surrounded him, brandishing semiautomatic weapons.

On Saturday, Mohammed denounced the Pakistani and U.S. governments and praised Osama bin Laden during a rally in the area attended by 5,000 pro-Taliban and al-Qaida tribesmen.

Before burial, the remains of at least 50 people were laid on traditional wooden beds placed side by side in rows and covered with colored blankets. Locals walked among the beds and offered prayers.

"The government has launched an attack during the night, which is against Islam and the traditions of the area," Siraj ul-Haq, a Cabinet minister from the North West Frontier Province, told the AP. Ul-Haq said he would resign in protest.

"We heard helicopters flying in and then heard bombs," said one of the villagers, Haji Youssef. "We were all saddened by what we have seen."

Thousands of people traveled from nearby villages to inspect the destroyed madrassa, some crying and others chanting "Long live Islam." The blast leveled the building, tearing mattresses and scattering Islamic books, including copies of the Quran.

In the nearby town of Khar, some 2,000 tribesmen and shopkeepers marched through the main street. "Death to Musharraf, Death to Bush," the procession chanted.

The attack happened about two miles from Damadola, where in January a U.S. Predator drone fired a missile that purportedly targeted -- and missed -- al-Zawahri, but killed several Al Qaeda members and civilians instead.

The attack coincided with Monday's planned signing of a peace deal between Bajur tribal leaders and the military aimed at stopping militants operating in the area and crossing into Afghanistan.

U.S. and Afghan officials criticized a similar deal for North Waziristan, saying it could turn the area into a terrorist haven. Pakistani authorities say the deal returns power back to traditional tribal elders, but the military also stresses that it will crack down if militant activity resumes.

Pakistan became a key U.S. ally in its war on terror after the Sept. 11, 2001, attacks and has deployed about 80,000 soldiers in the poorly marked Pakistan-Afghan frontier, where bin Laden is believed to be at large.