Pakistan's President Pervez Musharraf has declared emergency rule and suspended the country's constitution.
He defended his actions in a national address, saying he was curbing a rise in extremism in Pakistan.
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".
In a lengthy televised speech late on Saturday, Mr Musharraf said the situation had forced him into making "some very painful decisions".
"I suspect that Pakistan's sovereignty is in danger unless timely action is taken," he said.
He insisted his decisions were made for the benefit of Pakistan.
"Extremists are roaming around freely in the country, and they are not scared of law-enforcement agencies," the president said.
As well as defending emergency rule to the Pakistani people, Gen Musharraf also appealed directly to his Western allies for patience.
"Kindly understand the criticality of the situation in Pakistan and around Pakistan. Pakistan is on the verge of destabilisation," he said.
"Inaction at this moment is suicide for Pakistan and I cannot allow this country to commit suicide."
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