Violent clashes have broken out in Pakistan between security forces and supporters of former prime minister Imran Khan after he was arrested on Tuesday.
Protests are erupting nationwide, and at least one person has been killed in the city of Quetta.
The United States and UK have called for adherence to the “rule of law”.
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.
Many analysts believe Mr Khan’s election win in 2018 happened with the help of the military. Now in opposition, he is one of its most vocal critics, and analysts say the army’s popularity has fallen.
Footage from Lahore posted on Twitter appeared to show a crowd breaking into the military corps commander’s house destroying furniture and belongings inside.
Speaking from Washington, the US Secretary of State Antony Blinken said he wanted to make sure that “whatever happens in Pakistan is consistent with the rule of law, with the constitution”.
UK Foreign Secretary James Cleverly, speaking alongside Blinken, noted that Britain enjoyed “a longstanding and close relationship” with Commonwealth member Pakistan, and wanted to “see the rule of law adhered to”.
On Tuesday evening, supporters of Imran Khan gathered outside the Pakistan High Commission in London to protest against his arrest.
Mr Khan was ousted as PM in April last year and has been campaigning for early elections since then.
Speaking to the BBC’s Newshour, Mr Khan’s spokesman, Raoof Hasan, said he expected “the worst” and that the arrest could plunge the country “into chaos and anarchy”.
“We’re facing multiple crises. There is an economic crisis, there is a political crisis, there is a cost of livelihood crisis and consequently this occasion will be a catharsis for them to step out and I fear a fair amount of violence is going to be back,” he said.
A member of Mr Khan’s legal team, Raja Mateen, said undue force had been used against him at the court.
“Mr Khan went into the biometric office for the biometrics. The rangers went there, they broke the windows, they hit Mr Khan on the head with a baton,” said Mr Mateen.
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.
People pulled down street signs and parts of overpasses, lit fires and threw stones. During the hour or so that the BBC was there, no police or authorities were visible.
A statement from the inspector general of Punjab police said the arrest of Mr Khan had been ordered because he was accused of “corruption and corrupt practices”.
The case involves allegations over the allotment of land in the so-called Al-Qadir Trust, which is owned by Mr Khan and his wife, Dawn newspaper reported.
Mr Khan, who is being held at an undisclosed location, denies breaking any law.
In a video message filmed as he travelled to Islamabad – and released by the PTI before his arrest – Mr Khan said he was ready for what lay ahead.
“Come to me with warrants, my lawyers will be there,” he said. “If you want to send me to jail, I am prepared for it.”