Pakistani opposition leaders and activists have been detained in the wake of President Pervez Musharraf's decision to declare emergency rule.
Armed police have been deployed on the streets of Islamabad
The acting head of the party of exiled former PM Nawaz Sharif was arrested, senior lawyers have been detained and the country's chief justice sacked.
PM Shaukat Aziz said that hundreds of people had been held, and the emergency would last "as long as is necessary".
Scheduled elections could be delayed for up to a year, he added.
But no decision had been made over the date of any election, he added, insisting the government remained committed to the democratic process.
Speaking late on Saturday, Gen Musharraf defended his decision, saying he could not allow the country to "commit suicide".
He said Pakistan was in a crisis caused by militant violence and a judiciary which had paralysed the government.
The moves came as the Supreme Court was due to rule on the legality of Gen Musharraf's October election victory.
Threat of force
Police and paramilitaries manned checkpoints around the parliament and presidential palace in the capital, Islamabad, on Sunday morning.
After a calm start, a few dozen people staged a brief protest near the parliament building before police moved in to break up the gathering.
Several people were dragged away and arrested, reports the BBC's Syed Shoaib Hasan, in the city.
More protests are expected throughout the rest of the day, he adds, with police appearing ready to use force against unauthorised demonstrations.
Tough new media restrictions are controlling the news available throughout Pakistan: all non-state TV stations and some radio channels, including international services such as BBC World TV, have been taken off air.
Independent newspapers have been allowed to continue publishing, but Gen Musharraf's decree severely limits what they can report.
Local newspapers and key opposition leader Benazir Bhutto accused Gen Musharraf of bringing in martial law without formally declaring it.
But Pakistan's attorney general said the prime minister and parliament remained in place and the civilian government would continue to function.
Speaking at a news conference, Prime Minister Shaukat Aziz said there had been 400 to 500 "preventative arrests" since the emergency was declared.
Before he was taken away by police in the central city of Multan, Javed Hashmi, of Nawaz Sharif's Pakistan Muslim League, said Gen Musharraf would pay a price for his decision to restrict freedoms.
"Musharraf's days are numbered. Time has come to end the political role of the army," he said.
Constitutional safeguards on life and liberty curtailed
Police get wide powers of arrest
Suspects can be denied access to lawyers
Freedom of movement restricted
Private TV stations taken off air
New rules curtail media coverage of suicide bombings or militant activity
Chief justice replaced, others made to swear oath of loyalty
Supreme Court banned from rescinding emergency order
Ms Bhutto, who recently returned to Pakistan from self-imposed exile, flew back to Karachi from a trip abroad upon hearing news of Gen Musharraf's decision.
She confirmed that troops were not surrounding her Karachi home, contrary to some earlier reports, and laid out her demands for the holding free and fair elections.
"We the political parties are calling for the restoration of the constitution, and for the holding of the elections under an independent election commission," she told the BBC.
There is no word yet whether she plans to enter dialogue with the president or to lead opposition to his rule.
In a TV address on Saturday evening, Gen Musharraf explained his decision, saying the current situation had forced him into making "some very painful decisions".
Pakistan has been engulfed in political upheaval in recent months, and the security forces have suffered a series of blows from pro-Taleban militants opposed to Gen Musharraf's support for the US-led "war on terror".
"Extremists are roaming around freely in the country, and they are not scared of law-enforcement agencies," the president said.
"Inaction at this moment is suicide for Pakistan and I cannot allow this country to commit suicide."
Following the announcement of emergency rule, the country's chief justice was replaced and the Supreme Court surrounded by troops.
Chief Justice Iftikhar Chaudhry and eight other judges refused to endorse the emergency order, declaring it unconstitutional, resulting in Mr Chaudhry's dismissal.
Gen Musharraf moved quickly to appoint a new chief justice
The Supreme Court was to decide whether Gen Musharraf was eligible to run for re-election last month while remaining army chief.
Fears were growing in the government that the court could rule against Gen Musharraf.
Pakistani lawyers announced they would strike on Monday in protest at the president's decision.