Fearing unrest could spread, Pakistan stepped up security in its capital Tuesday and detained three Muslim clerics who organized anti-American demonstrations. Three people, including a 13-year-old boy, died in new violence.

The crackdown followed a daylong riot Monday in the Islamic fundamentalist stronghold of Quetta, where protesters burned cars and a police station and looted a bank to demand an end to the U.S.-led attack on neighboring Afghanistan.

That was the most violent protest in Pakistan since the start of the bombing against strongholds of Usama bin Laden and his allies in Afghanistan's ruling Taliban militia — admired by Pakistani religious extremists who object to President Gen. Pervez Musharraf's support of the international anti-terrorism coalition.

Despite large, sometimes raucous protests in some cities near the Afghan border, most of Pakistan has been quiet. Musharraf wants to keep it that way.

"In an Islamic society, there is no room for extremism and violence against any other religion or group," he said Tuesday. He said support of his government now is part of "supreme national interests."

Security forces placed sandbags around police positions at key government installations here in the capital Tuesday, and military vehicles with machine guns were seen patrolling major streets.

Three people were shot and killed killed when up to 400 baton-wielding Afghan refugees attacked a police station in the small town of Kuchlak near Quetta, authorities said. One of the victims was identified by doctors at the hospital in Quetta as 13-year-old Hamid Ullah.

The police superintendent in Quetta, Abid Ali, said the refugees first attacked a bank and post office in Kuchlak. They turned to the police station when officers tried to break them up.

Quetta police chief Abid Ali said 75 people were arrested. Protester Mohammed Aman said the demonstration was peaceful and the police gunfire unprovoked.

In the eastern city of Lahore, several hundred pro-Taliban demonstrators stoned police, blocked roads and chanted slogans against President Bush and Musharraf for his support of the United States.

Protesters sounded similar themes without violence in two other cities — 5,000 in Karachi, Pakistan's largest city, and several hundred in the northwestern city of Peshawar near the Khyber Pass border area.

The actions against protest leaders are part of a sweeping campaign by Musharraf to prevent moves to destabilize his government because of its support for the bombing campaign. He has also reshuffled the senior ranks of the armed forces, which he leads.

Two of the clerics who drew police concern were placed under house arrest for three months, the Interior Ministry said.

Maulana Fazal-ur Rehman, leader of the Jamiat Ulema-e-Islam party, had already been under house arrest twice since Sunday after leading demonstrations in the days before the strikes began. Azam Tariq, chief of the Sipah-e-Sahaba party, was detained at the Lahore airport en route to Islamabad for a meeting of religious leaders and escorted back to his home in southern Pakistan.

The whereabouts of a third cleric, Samiul Haq, the pro-Taliban leader of the Afghan Defense Council, were not known. Police would only confirm he had been detained, and his spokesman, Yousaf Shah, said he was taken "to an unknown place, and we are not in contact with him."

Spokesmen for the parties condemned the action against their leaders. Jamiat Ulema-e-Islam spokesman Riaz Durrani, whose party claims millions of followers, gave the government 24 hours to release Rehman or "be responsible for the consequences."

Tariq's spokesman, Maulana Mujibur Rehaman, had a message for Musharraf: Talk with the party leaders and explain how Pakistan benefits from supporting the United States.

"You convince us, and we will stop agitation. But if we convince you, you should support the Taliban," he said.

Meanwhile, the United Nations said no one was injured Monday when a mob in Quetta attacked its compound there, stoning the office of the U.N. High Commissioner for Refugees and torching the UNICEF building.