Pakistan's President Pervez Musharraf has declared emergency rule and suspended the country's constitution.
Chief Justice Iftikhar Chaudhry, who condemned the moves, has reportedly been sacked and is being confined to the Supreme Court with 10 other judges.
It comes as the court was due to rule on the legality of Gen Musharraf's re-election victory in October.
The Court was to decide whether Gen Musharraf was eligible to run for election last month while remaining army chief.
The BBC's Barbara Plett reports from Islamabad that fears have been growing in the government that the Supreme Court ruling could go against Gen Musharraf.
Former Prime Minister Benazir Bhutto, who recently returned to the country after years of self-exile to lead her party in the elections, was in Dubai on a personal visit when news of the declaration broke.
However, she immediately boarded a flight back to Pakistan in response, landing in Karachi.
Her return from self-imposed exile last month came about with the co-operation of Gen Musharraf.
Our correspondent says in the changed circumstances she will have to decide whether she is returning to lead the opposition against the president, or should wait on the sidelines in the hopes of securing an agreement with him.
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".
Pakistan's Cabinet is currently meeting to approve Gen Musharraf's declaration of emergency rule. He is expected to address the nation later.
A leading lawyer and opposition figure, Aitzaz Ahsan, told reporters that he had been detained as the emergency powers were invoked. "They have served me a detention order for 30 days," Mr Ahsan, president of the Supreme Court Bar Association, was quoted by Reuters as saying.
"One man has taken entire nation hostage... Time has come for General Musharraf to go."
Parliamentary elections are due in January - it is not clear whether they will go ahead.
Copyright 2007 BBC