Police, soldiers and paramilitary guards have been posted in key locations around Pakistan following the announcement of emergency powers.
Security was reinforced near Islamabad's presidential palace. President Pervez Musharraf declared the emergency before the Supreme Court ruled on his presidency.
Human rights activists quickly gathered to protest against Gen Musharraf's move. Authorities have already arrested up to 500 people around the country, Prime Minister Shaukat Aziz said.
Police have been given new powers to detain potential troublemakers, such as human-rights activists protesting against the emergency.
Javed Hashmi, the acting chief of Pakistan Muslim League-N (of former PM Nawaz Sharif) was among the public figures arrested after emergency rule was declared.
Opposition leader Benazir Bhutto, of the PPP, swiftly condemned President Musharraf's move. She was not arrested, unlike other leading political figures in Pakistan.
Newspaper vendors were preparing for brisk business on Sunday morning, as newspapers reported the introduction of emergency rule.
Pakistanis watched the address by President Musharraf justifying the introduction of emergency rule. He blamed militants and the judiciary for paralysing government.