Pakistani opposition leader, Fawad Chaudhry, was arrested on Wednesday morning on charges of sedition after a press conference outside Imran Khan's residence in Lahore.
Faisal Hussain, Chaudhry's brother and a lawyer, confirmed the arrest on Twitter.
Videos on social media showed Chaudhry being taken to the Cantt court in Lahore under tight security by officials of the Punjab Counter Terrorism Department and police.
This has been a tense time, with PTI supporters gathering amid speculation that Khan himself might be arrested.
The government of Prime Minister Imran Khan has been cracking down on opposition and critical journalists since coming to power.
Sedition is a broad charge that critics say is being increasingly used to stifle opposition.
Chaudhry has been a fierce critic of the government, particularly over its handling of the economy.
The opposition has accused the government of stifling dissent and suppressing freedom of speech in the country.
However, there are conflicting reports over the reasons for Chaudhry's arrest.
According to Dawn, an English-language newspaper in Pakistan, Chaudhry was arrested for making "derogatory remarks" about a judge, whereas other sources suggest the arrest was related to his criticism of the government.
Human Rights Watch has called for an immediate end to the government's "relentless assault on free speech" and "arbitrary arrests" of opposition figures.