Protests are ongoing nationwide, and at least one person has been killed, following the arrest of Imran Khan.
Mr Khan was arrested by security forces at the High Court in the capital, Islamabad.
Dramatic footage showed dozens of officers arriving and detaining the 70-year-old, who was bundled into a vehicle and driven away.
He was appearing in court on charges of corruption, which he says are politically motivated.
Mobile data services in the country were suspended on the instructions of the interior ministry on Friday as protests grew, many of them taking place in front of army compounds.
Pakistan's army plays a prominent role in politics, sometimes seizing power in military coups, and, on other occasions, pulling levers behind the scenes.
Mr Khan was ousted as PM in April last year and has been campaigning for early elections since then.
General elections are due to be held later this year.
Mr Khan's Pakistan Tehreek-e-Insaf (PTI) party called on its supporters to protest. In the hours after he was detained, violence was reported from cities including Lahore, Karachi and Peshawar.
On the streets of Islamabad, hundreds of protesters blocked one of the main highways in and out of the capital.
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