Mr. Khan, a former cricket star, has staged a comeback since being ousted, openly challenging the powerful military. His detention raises fears of mass protests.
Pakistan’s ousted prime minister, Imran Khan, was arrested on corruption charges Tuesday, in a major escalation of a political crisis that has engulfed the country over the past year and that raises the prospect of mass unrest by his supporters.
The arrest intensified a showdown between the powerful Pakistani military and Mr. Khan, and brought the country into uncharted political territory. While Pakistani leaders have faced arrest before, never has anyone like Mr. Khan so directly — and with mass popular support — challenged the military, which for decades has been the invisible hand wielding power behind the government.
Political tensions have been building for months as Mr. Khan, a populist former cricket star who was removed from office last year, has accused the military and the current government of conspiring against him. Both military and government officials deny those claims.
Mr. Khan was at a court hearing in Islamabad when he was arrested by paramilitary troops, and he remained in custody on Tuesday, officials said. A video of the arrest, circulating on social media, shows Mr. Khan being led into a law enforcement vehicle, surrounded by a throng of security officers in riot gear. Mr. Khan was arrested in connection with a land transfer for a university, officials said — one of several corruption cases he is facing.
His political party called for demonstrations soon after his arrest, and protests erupted in several cities.
In Lahore, throngs of Mr. Khan’s supporters ransacked the official residence of an army commander. Hundreds of protesters also gathered outside the army headquarters in Rawalpindi, just outside Islamabad. And in the port city of Karachi, the police fired tear gas to disperse crowds that blocked the city’s main thoroughfare, and protesters burned a police vehicle, a prison van and a paramilitary troops’ checkpoint.
The unrest offered a stark reminder of the turbulent political scene in Pakistan, a nuclear-armed nation of 230 million people that has struggled with instability and military coups since its founding 75 years ago. The military has ruled for over half of that history, and even under civilian governments, military leaders are seen as the force responsible for ushering political dynasties into and out of power.
Mr. Khan — whose party commands significant loyalty across the country — has made a stunning political comeback in the wake of his ouster last year. Tens of thousands have thronged his rallies, at which Mr. Khan and others have called for fresh elections and accused Pakistan’s military establishment of orchestrating his removal.
Prosecutors have in recent months opened dozens of court cases against Mr. Khan, including on charges of terrorism and corruption, and he has repeatedly faced threats of arrest after failing to appear in court.
Mr. Khan and his supporters deny the charges, characterizing them as a misuse of the justice system by the government, led by Prime Minister Shahbaz Sharif, and the military to keep him out of politics. Political and military leaders deny those claims.
The tensions surrounding Mr. Khan broke into violence in November, when he was wounded during a rally after an unidentified man opened fire on his convoy, in what aides have called an assassination attempt. In recent months, Mr. Khan has accused a senior intelligence official of playing a role in that shooting.
On Monday, military officials offered a sharp rebuke to those accusations, releasing a statement saying Mr. Khan had targeted military and intelligence officials with “insinuations and sensational propaganda” to further his political agenda.
The news release, a rare public statement from the military leaders directed at a political leader, underscored the severity of the clash between them and Mr. Khan.
His arrest is “about Khan crossing the military’s ‘red line’ with his recent comments against officials in the military and intelligence services,” said Madiha Afzal, a fellow at the Brookings Institution. “It is about Khan’s escalating confrontation with the military establishment over the last year, and the fact that the latter sees Khan as an existential threat.”
Mr. Khan was arrested in connection with a case involving the transfer of land for Al-Qadir University, near Islamabad, officials said. Mr. Khan is accused of granting favors to Malik Riaz Hussain, a powerful real estate tycoon, with the university getting land and donations in return.
The National Accountability Bureau, Pakistan’s anticorruption organization, said it had sent multiple notices to Mr. Khan that were ignored. Officials said he is now being held at one of the bureau’s offices in Rawalpindi, in northern Pakistan.
He will be presented before a court Wednesday, officials said.
The drama surrounding Mr. Khan seems only to have buoyed his popularity, analysts say, highlighting his ability to outmaneuver Pakistan’s typical playbook for sidelining leaders who have fallen out of favor with the military.
Over the summer, his party, Pakistan Tehreek-e-Insaf, or P.T.I., won sweeping victories in local elections in Punjab — a province that has often served as a bellwether for national politics — and in Karachi.
Those victories were also seen as a response to worsening economic conditions that the government has struggled to address, and as a repudiation of the military establishment
But they have drawn a crackdown on Mr. Khan’s supporters that many view as a coordinated effort by the authorities to dampen his political prospects.
Journalists known to be sympathetic to Mr. Khan say they have been harassed by the authorities. Live broadcasts of his speeches have been banned from news networks. A mainstream channel, ARY News, was forced off the air after airing an interview with one of Mr. Khan’s top aides in which he made anti-military remarks.
The crisis has flipped the script for Mr. Khan, who benefited from a close relationship with the military when he was elected prime minister in 2018. At the time, his rivals claimed that the authorities had waged a campaign of coercion and intimidation that deterred any opposition and ensured Mr. Khan’s success.
Military officials have denied those accusations and maintained that the institution has adopted a “neutral” position in the current political crisis. The military appeared to withdraw its support for Mr. Khan at the beginning of last year, paving the way for lawmakers in Parliament to remove him with the no-confidence vote.
But Mr. Khan has retained widespread popularity, a sign that the authorities’ traditional methods may not be enough to silence a populist politician in the era of social media, analysts say.
Now, many fear Mr. Khan’s arrest will worsen the turmoil that has embroiled the country in recent months. Before Tuesday, his aides were warning that his detention would set off mass unrest.
“His supporters have demonstrated their capacity to turn up in large numbers and bring life to a standstill,” said Zahid Hussain, a political analyst and author based in Islamabad.
Soon after the arrest, official social media accounts for Mr. Khan’s political party, released a prerecorded message from Mr. Khan, in which he directs party workers to stage protests across the country after his arrest.
Many rallied behind that appeal on Tuesday, and protests are expected to continue this week — raising the specter of possible violent clashes between the police and Mr. Khan’s supporters.
“We are not afraid of tear gas and batons,” said Muhammad Shafiq, a 24-year-old student, who was among those protesting in Karachi. “For us, there is nothing more important than Mr. Khan.”
Zia ur-Rehman contributed reporting.