Pakistan's President Pervez Musharraf has declared emergency rule and has suspended the country's constitution.
A proclamation said the judiciary were interfering with the government and that Islamic militants were increasingly threatening security.
In an address on state-run television, President Musharraf said that terrorism and extremism had reached an extreme level and that it was time to take 'painful decisions'.
Pakistan has witnessed mounting political uncertainty and a wave of violence by pro-Taliban and al-Qaeda linked militants.
It is understood paramilitary troops and police have surrounded the Supreme Court in Islamabad.
The court is due in the coming days to give a verdict on the legality of President Musharraf's victory in the 6 October presidential election.
All land and mobile telephone lines went down shortly after the declaration while private television channels were taken off the air.
President Musharraf, a key US ally who seized power in a coup in 1999, may now also delay general elections due in January, which were seen as a key step in moving nuclear-armed Pakistan towards full civilian democracy.
Meanwhile, former Pakistan Prime Minister Benazir Bhutto described the imposition of emergency rule by President Musharraf as 'mini-martial law' and vowed that her party will protest against it.
Ms Bhutto had been in negotiations with President Musharraf over Pakistan's transition to civilian led-democracy.
The US Secretary of State, Condoleezza Rice, has described Pakistan's declaration of emergency rule as 'highly regrettable'.
She said she hoped Pakistan's intention was to have free and fair elections.
