Prime Minister Shehbaz Sharif's government also deployed paramilitary rangers and additional police and shut schools in Islamabad and the nearby city of Rawalpindi after Khan's Pakistan Tehreek-e-Insaf party refused to withdraw its call for the protest.
Videos posted online showed police placing shipping containers on bridges and roads on a key highway in the northwest Khyber Pakhtunkhwa province, where Khan's party holds power.
Khan's supporters plan to march from the province to Islamabad on Friday, defying a ban on rallies imposed this week.
Police reportedly arrested some party supporters from Islamabad and swung batons and used tear gas to prevent the rallygoers entering the capital from the northwest.
Supporters are demanding the release of jailed former PM Imran Khan. (AP PHOTO)
Khan, Sharif's main political rival, has been in prison for more than a year in connection with more than 150 police cases.
He remains a popular figure despite the cases, which critics and his party say are politically motivated.
Khan was ousted in 2022 through a no-confidence vote in parliament and arrested in 2023 after a court handed him a three-year jail sentence in a graft case.
The suspension of mobile phone service in Islamabad and Rawalpindi on Friday disrupted communications and affected basic services such as online banking and ride and food delivery services.
On Thursday, Interior Minister Mohsin Naqvi told Khan's party to cancel the planned protest and warned there would be "no leniency" if the ban on protests was defied.
A message on Khan's social media account urged supporters to join the protest.
"The tyrants in power want to terrorise us," it said.
"So go forth fearlessly, and remember if you still hesitate, to step forward and truly liberate yourself."
Sharif's government says Khan's party wants to weaken the country's economy by staging violent protests despite the threat of attacks by the Pakistani Taliban, who have stepped up attacks in recent years.
On Friday, supporters of Khan gathered in Swabi, a city in Khyber Pakhtunkhwa province, to begin a march towards Islamabad.
Officials said the provincial government had mobilised heavy machinery to remove the shipping containers placed on the highway leading to the capital.
