All the private television channels have been cut off. Only two state channels are on. Private television said earlier army units were entering Islamabad to take control over the center of the city and the government quarters.
The possibility of a state of emergency had been discussed earlier in view of more frequent terrorist attacks in the country and intensified activity of militants near the border with Afghanistan.
On Friday, the Supreme Court of Pakistan rescheduled hearings on the legitimacy of the October 6 elections, which saw President Pervez Musharraf reelected, from November 12 to November 5.
The Supreme Court previously said the official election results would only be announced after a complaint from two opposition candidates had been considered. The complaint said Musharraf had no right to be reelected as he had retained the post of Army commander.
Shortly before the October elections, Musharraf, who came to power in a bloodless coup in 1999, said he would give up the post if reelected and become a civilian president.
Pakistan will hold parliamentary elections in early January 2008.