Pakistan opposition leader Imran Khan can be held for eight days for questioning, a judge ruled on Wednesday after the former prime minister was dragged from a courtroom and arrested over corruption charges.
He was indicted for unlawfully selling state gifts during his premiership between 2018-22.
His arrest led to violent clashes and his supporters protested outside the military’s headquarters on Tuesday. More protests erupted on Wednesday.
The indictment followed a decision by the Election Commission of Pakistan last October which found Mr Khan guilty of illegally selling state gifts and barred him from holding public office until the next election.
He has denied any wrongdoing and his supporters were planning to march to Islamabad, where the former prime minister is in custody in a corruption case, on Wednesday.
There were concerns that that could lead to more clashes with security forces.
Mr Khan was arrested on Tuesday by paramilitary troops on the orders of the National Accountability Bureau, an autonomous anti-corruption agency, in relation to a case involving a land deal.
He was moved to its headquarters in the garrison town of Rawalpindi for questioning late into the night as angry protests erupted across Pakistan.
The Islamabad High Court said late on Tuesday that the arrest was legal, according to Mr Khan’s lawyer, Intazar Hussain Panjutha.
Mr Khan’s legal team intends to challenge his detention and any request from the NAB to keep him in custody, Mr Panjutha added.
The arrest came a day after the country's powerful military rebuked Mr Khan for repeatedly accusing a senior military officer of trying to engineer his assassination and the former armed forces chief of being behind his removal from power last year.
In response, his Pakistan Tehreek-e-Insaf party called for a “shutdown” across the country, with Mr Khan's supporters clashing with police in many cities and storming military buildings in Lahore and Rawalpindi, according to witnesses and videos.
Supporters in north-western Khyber Pakhtunkhwa province were asked to gather early on Wednesday in Swabi city to leave for Islamabad as part of a convoy, the party wrote on Twitter.
Party leaders asked workers to continue protests but not take the “law in their hands”, they wrote on Twitter.
A police spokesman told Reuters that Mr Khan would not be taken to court and his scheduled hearing would instead take place at the location where he is being held.
The former cricket star, 70, has regularly criticised members of the military, Pakistan’s most powerful institution, since he was ousted as prime minister in April last year.
'More violence … on the way'
A prolonged detention for Mr Khan, whose popularity has been surging ahead of an election that must be called by mid-August, threatens to destabilise Pakistan just as it looks to secure more cash from the International Monetary Fund to avoid default.
“More protests, more disruption, more violence, and more political and economic dysfunction is on the way,” Hasnain Malik, a strategist at Tellimer in Dubai, told Bloomberg.
“Taking the painful decisions the economy needs and holding orderly elections will become even harder now.”
As news of Mr Khan’s detention spread, his supporters in several cities took to the streets and clashed with security forces, who fired tear gas and tried to beat them back.
A video from Mr Khan’s party showed some supporters wielding sticks and face masks entering the army’s headquarters in Rawalpindi and shouting angrily.
Protesters also stormed the official residence of the top regional commander in Lahore, a rare breach of an area controlled by the military. Both sites were later cleared.
Mr Khan’s party reported at least four people were killed and 20 injured in separate clashes. Protesters also set fire to at least three buildings across Pakistan, while police said dozens of supporters were arrested.
The Interior Ministry suspended mobile broadband services in parts of the country, while complaints of slow internet spread on social media.
It is unclear whether Mr Khan will be released before the election. The NAB in the past has detained former prime ministers and top government officials for lengthy periods of time before they have been released and ultimately found innocent.
During Mr Khan’s administration, Prime Minister Shehbaz Sharif, who was then in opposition, was arrested by the NAB and kept in custody for nearly seven months.
Mr Sharif’s government attempted but failed to arrest Mr Khan over charges in several cases in recent months, in part to avoid stoking public anger that could further increase his popularity.
Interior Minister Rana Sanaullah said on Tuesday the government had no control over the NAB.
“Given that Khan was detained by paramilitary forces, this was likely the military acting directly,” Michael Kugelman, director of the South Asia Institute at the Washington-based Wilson Centre, told Bloomberg.
“But if so, the civilian leadership, given their long and ugly vendetta with Khan, surely backed the move.”
