Pakistan's President Pervez Musharraf declared a state of emergency, suspended the constitution and removed the nation's top judge Saturday.
Hours after security forces blocked off key streets around government buildings and imposed tough curbs on the media, General Musharraf appeared on national television to defend his decision.
He blamed rising violence from Islamic militants and a series of judicial decisions for demoralizing law enforcement officials, setting known terrorists free, and undermining his efforts to move the country toward democracy.
But General Musharraf's critics say the move undermines democracy and will inflame militants.
Late Saturday, some 50 protesters gathered at the police barriers outside the President's House, chanting "Go Musharraf, Go." Security forces did not intervene.
Earlier, troops entered the Supreme Court in Islamabad and took away chief justice Iftikhar Chaudhry, whose suspension earlier this year triggered nationwide protests. The court was to rule in a few days on the validity of General Musharraf's re-election as president last month by Parliament.
Early Sunday, there were reports of gunfire in Pakistan's largest city, Karachi, Benazir Bhutto's stronghold. She is at her family home in the port city, with a large protective force of police outside.
Ms. Bhutto, the former prime minister who recently returned to her homeland from eight years of exile, said General Musharraf's actions had placed Pakistan under martial law, and she vowed to oppose his emergency rule.
Police rounded up many opposition politicians and human rights activists around the country.
Those detained or held under house arrest include Imran Khan, the internationally known former cricketer who leads a small opposition party; and the attorney leading the legal challenge to General Musharraf's re-election.
Under President Musharraf's provisional constitutional orders, state television said, government ministries and Parliament will continue to function during the state of emergency.
The president said he remains committed to holding parliamentary elections, but he did not indicate whether he would try to keep to the existing schedule for a vote in January.