Heavily-armed police rounded up suspected militants in religious schools and mosques just hours before President Gen. Pervez Musharraf was to speak on measures to curb religious extremism, terrorism and violence.

The nationwide sweep began Friday and continued through Saturday, with the arrests of at least 250 Muslim extremists, senior Interior Ministry officials said.

The crackdown was a precaution against any violent reaction by extremist Islamic groups after Musharraf's speech, said a senior police official, who asked not to be identified.

The arrests also come against a backdrop of high tensions between Pakistan and India, which accuses Pakistani intelligence and two Islamic groups of being behind a Dec. 13 attack on its Parliament. Pakistan denies involvement.

Fourteen people, including the five assailants, were killed in the parliament attack. The incident prompted the two South Asian nuclear rivals to send troops to the frontier in disputed Kashmir -- their largest military buildup since 1971.

Most of those arrested belonged to the extremist Sunni Muslim group Sipah-e-Sahaba, or Guardians of the Friends of the Prophet, and the Shiite Muslim Tehrik-e-Jafria, or Movement for the Imposition of Shiite Law. Some members of the Sunni Tehrik, or Movement, also were arrested, police said. Islamic anti-Indian groups tied to unrest in Kashmir also were detained throughout the country.

In the restive port city of Karachi, police raided and searched three religious schools and two mosques run by the Sipah-e-Sahaba in the eastern Gulshan-e-Iqbal neighborhood for suspected terrorists and weapons, as well as several private homes. They arrested 74 people, the police official said.

There was no word whether any weapons were seized. Karachi, a teeming city of more than 12 million people, is Pakistan's main commercial hub and the capital of Sindh province.

Police arrested more than 150 others in similar raids in over a dozen other cities, among them Behawalpur, home of Jaish-e-Mohammad chief Maulana Masood Azhar who is accused by India of terror, and Jhang and Akora Khatak, homes to pro-Taliban clerics Maulana Azam Tariq, and Maukana Samiul Haq. Also raided was Dera Ismail Khan, home town of Maulana Fazle ur-Rehman, another pro-Taliban leader detained recently by Pakistani authorities.

Police blame the Sipah-e-Sahaba and Tehrik-e-Jafria for most of Pakistan's sectarian bloodletting, which kills hundreds of people each year.

Musharraf, who banned the armed wings of the two groups last year, has vowed to curb religious extremism and violence. In August, he enacted a special anti-terrorism law allowing bans of groups promoting religious hatred and violence.

Besides targeting the two groups, the government has also shut offices of militant Islamic movements, including those waging a secessionist war in the disputed region of Indian Kashmir. They have been barred from displaying banners, flags and collecting donations.

The officer said police have been ordered to remove flags and banners of all the militant religious, ethnic and political groups.

The United States and its allies have been urging Musharraf for swift and tough action against Islamic extremists. They say a crackdown on militants would help in easing tensions with India.

The Associated Press contributed to this report.