ISLAMABAD, Pakistan (CNN) -- Pakistani President Gen. Pervez Musharraf said he declared a state of emergency and martial law Saturday because Pakistan is at a "critical and dangerous juncture."
President Pervez Musharraf explains his actions in a televised address Saturday.
The nation is going through "some very rapid changes," Musharraf said in a televised address to the nation after declaring martial law.
Despite immediate condemnation from within and outside his nation, he insisted that his actions are for the good of the country and the move is to stabilize unrest.
Musharraf had earlier in the day issued an order proclaiming the emergency and suspending the nation's constitution, according to a statement read on state television, and declaring martial law.
The Supreme Court declared the state of emergency illegal, claiming Musharraf had no power to suspend the constitution, Chief Justice Iftikhar Mohammed Chaudhry said.
A senior Pakistani official said the emergency declaration will be "short-lived," and will be followed by an interim government. Martial law is a way to restore law and order, he said.
Shortly afterward, Chaudhry was expelled as chief justice, his office said. Troops came to Chaudhry's office to tell him.
The government appointed Justice Abdul Hameed Dogar as the new chief justice, according to state television.
It was the second time Chaudhry was removed from his post. His ousting by Musharraf in May prompted massive protests, and he was later reinstated. See a timeline of upheaval in Pakistan »
In Islamabad, troops entered the Supreme Court and were surrounding the judges' homes, according to CNN's Syed Mohsin Naqvi.
Supreme Court sources said some judges who were not in Islamabad were not at their homes, and it was not known whether they had been arrested.
Aitzaz Ahsan, a leading Pakistani attorney and president of the Supreme Court Bar Association, was arrested at his home. A former interior minister, Ahsan represented Chaudhry the first time he was forced to leave his post.
The White House called Musharraf's action disappointing.
"President Musharraf needs to stand by his pledges to have free and fair elections in January and step down as chief of army staff before retaking the presidential oath of office," said National Security Council spokesman Gordon Johndroe.
"All parties involved should move along the democratic path peacefully and quickly."
U.S. Secretary of State Condoleezza Rice, who is in Turkey for a conference with Iraq and neighboring nations, told CNN's Zain Verjee the developments in Pakistan were "highly regrettable."
The United States doesn't support any extra-constitutional measures taken by Musharraf, Rice said, urging restraint so violence can be avoided.
In Britain, Foreign Secretary David Miliband said in a statement, "We recognize the threat to peace and security faced by the country, but its future rests on harnessing the power of democracy and the rule of law to achieve the goals of stability, development and countering terrorism. I am gravely concerned by the measures adopted today, which will take Pakistan further from these goals."
Meanwhile, former Prime Minister Benazir Bhutto, who left Pakistan last week to visit her family in Dubai, arrived in Karachi on Saturday.
She returned to Pakistan last month, despite death threats, after several years in exile.
On October 18, upon her return, at least 130 people were killed when a suicide attacker tried to assassinate her. Bhutto was lightly wounded, but escaped largely unharmed.
Bhutto has pledged to help her party succeed in January's parliamentary elections. She hopes to gain a third term as prime minister, possibly under a power-sharing deal with Musharraf.
"The people of Pakistan will not accept it," Bhutto spokesman Farhatullah Babar said of the emergency declaration. "We condemn this move."
The declaration prompted a few hundred people to take to the streets in protest, but police and paramilitary groups blocked Islamabad's main roads and dispersed the crowds.
Earlier, private networks had reported the declaration was imminent as top officials huddled at Musharraf's residence in Rawalpindi. Shortly after that report, most media channels went off the air in an apparent blackout, although some flickered off and on.
The declaration could delay approaching parliamentary elections, according to CNN's Nic Robertson. It also could provide Musharraf with a reason to continue serving as the nation's military chief.
The nation's political atmosphere has been tense for months, with Pakistani leaders in August considering a state of emergency because of the growing security threats in the country's lawless tribal regions. But Musharraf, influenced in part by Rice, held off on the move. Video Watch a report on the volatile situation in Pakistan »
Since that time, Musharraf has faced a flurry of criticism from the opposition, who demanded he abandon his military position before becoming eligible to seek a third presidential term. Musharraf garnered a vast majority of votes in presidential elections last month; however, those results have not been certified by the nation's high court.
For weeks, the country has been coasting in a state of political limbo while the Supreme Court works to tackle legal challenges filed by the opposition that call into question Musharraf's eligibility to hold office. Some have speculated that the declaration of emergency was tied to rumors the court is planning to rule against Musharraf.
Musharraf, who led the 1999 coup as Pakistan's army chief, has seen his power erode since the failed effort to oust Chaudhry. His administration is also struggling to contain a surge in Islamic militancy.