ISLAMABAD — Police wielding assault rifles rounded up opposition leaders and rights activists today after Pakistan’s military ruler suspended the constitution, ousted the top justice, and deployed troops to fight what he called rising Islamic extremism.

President Musharraf, who seized power in a 1999 coup but had promised to hand over his army fatigues and become a civilian president this year, declared a state of emergency last night, dashing hopes of a smooth transition to democracy for the nuclear-armed nation.

“General Musharraf’s second coup,” read the headline in the Dawn daily. “It is martial law,” said the Daily Times.

Prime Minister Shaukat Aziz said the extraordinary measures would remain in place “as long as it is necessary.” He also said parliamentary elections could be postponed up to a year, but no such decision had been made.

Mr. Aziz also said that up to 500 opposition activists had been arrested in the last 24 hours.

Among those detained were the acting president of the party of a Nawaz Sharif, Javed Hashmi; a cricket star-turned politician, Imran Khan; the chairman of the independent Human Rights Commission of Pakistan, Asma Jehangir; and a former chief of the main intelligence agency and a staunch critic of General Musharraf’s support for the American-led war on terror, Hamid Gul.

Some 200 armed police stormed the rights commission office in Lahore today and arrested about 50 activists, a legal officer for the body, Mehbood Ahmed Khan.

“They dragged us out, including the women,” he said from the police station in the eastern city. “It’s inhuman, undemocratic and a violation of human rights to enter a room and arrest people gathering peacefully there.”

General Musharraf’s leadership is threatened by an Islamic militant movement that has spread from border regions to the capital, the reemergence of political rival and former prime minister, Benazir Bhutto, and an increasingly defiant Supreme Court, which was expected to rule soon on the validity of his recent presidential election win. Hearings scheduled for next week were postponed, with no new date set.

Attorney General Malik Mohammed Qayyum denied claims by Ms. Bhutto and others that General Musharraf had imposed martial law — direct rule by the army — under the guise of a state of emergency. He noted the prime minister was still in place and that parliament would complete its term, ending November 15.

In Islamabad, phone service that was cut yesterday evening appeared to have been restored by this morning. But transmissions by television news networks other than state-controlled Pakistan TV remained off the air.

Scores of paramilitary troops blocked access to the Supreme Court and parliament. Otherwise streets in the capital appeared calm, with only a handful of demonstrations. But one, attended by 40 people at the Marriott Hotel, was broken up by baton-wielding police.

“Shame on You! Go Musharraf Go!” the protesters shouted as officers dragged some out of the crowd and forced them to the ground. Eight were taken away in a van.

Western allies had urged General Musharraf not to take authoritarian measures despite recent his country’s recent turmoil.

Secretary of State Rice called for a return to democracy, as the American embassy urged citizens in Pakistan to remain at home and defer all nonessential travel. But a Pentagon spokesman, Geoff Morrell, said the emergency declaration “does not impact our military support” of the Muslim nation or its efforts in the war on terror.

In his televised address late yesterday, General Musharraf, looking somber and composed, said Pakistan was at a “dangerous” juncture, and that its government was threatened by Islamic extremists who were “imposing their obsolete ideas on moderates.”

The military ruler, wearing a black button-down tunic rather than his army uniform, also blamed the Supreme Court for tying the hands of the government by postponing the validation of his recent election. The court was expected to rule soon on opponents’ claims that General Musharraf’s October 6 victory was unconstitutional because he contested while army chief. He was elected by a Musharraf-led legislature.

Ms. Bhutto, who had traveled abroad following an October 18 suicide bombing that narrowly missed her but killed 145 others, immediately returned to the southern city of Karachi declared yesterday the “blackest day” in Pakistan’s history. “Judicial decisions have to be accepted even if they don’t suit you,” she said.

General Musharraf replaced the chief justice, Iftikhar Mohammed Chaudhry, who had emerged as the main check on the president. A lawyer who represented the judge, Aitzaz Ahsan, also was arrested.

Musharraf vowed to go ahead with parliamentary elections, originally due by January, but gave no timeline.

Deputy Information Minister Tariq Azeem said today he hoped the polls would go ahead soon: “But unfortunately everything has been put on the back burner. I can’t give you the exact date.”