After Pakistani President Pervez Musharraf declared a state of emergency, truckloads of soldiers arrived in the centre of the capital Islamabad.
Key institutions have been sealed off in Islamabad's administrative centre, including the Supreme Court and TV and radio stations.
Gen Musharraf gave a televised address late on Saturday, saying he was acting to curb a rise in extremism.
Former Prime Minister Benazir Bhutto cut short a visit to Dubai to return to Pakistan. She said her Pakistan People's Party would fight the emergency rule.
The Supreme Court - now blocked off - had been due to rule on the legality of Gen Musharraf's October re-election victory.
Crowds of both pro- and anti-Musharraf supporters have gathered in front of President House.
Earlier, checkpoints had been set up on the main roads leading into Islamabad amid increasing clashes in recent weeks between security forces and militants.
Militants said they had captured two police stations in Swat district, in the north-west, where pro-Taleban militants have dealt a series of blows to security forces.