The rally organized in Rawalpindi, a city adjoining Islamabad, and which should bring together thousands of people, comes in a particularly tense context. On November 3, the former Prime Minister was shot in the legs during a meeting. The attack came after months of a political crisis sparked by his ouster from power following a vote of no confidence in parliament against him in April.
Saturday is the most important stage of a "long march" organized by his party, Pakistan Tehreek-e-Insaf (PTI, Pakistan Movement for Justice) with the aim of putting pressure on the government to to secure early elections before Parliament's mandate expires next October.
"My life is in danger and despite my injuries, I am going to Rawalpindi for the nation", tweeted the PTI on Saturday morning, quoting Mr. Khan whose speech is expected in the evening. "My nation will come to Pindi (short for town) for me".
Rawalpindi, a garrison town, is home to the headquarters of the powerful Pakistani army.
"We hope that Khan will introduce reforms and that the situation will improve," testified to AFP Saghir Ahmed who closed his shop to attend the speech.
The 32-year-old tailor laments a dire economic situation with galloping inflation and a plummeting rupee that has made life "unbearable".
Early in the morning, PTI supporters streamed in from across the country sporting green and red party pennants and headed for the bulletproof paneled grandstand set a safe distance from the crowd.
The authorities have put in place a vast security device around the capital to prevent supporters of the former international cricket star, converted into politics, from walking on government buildings.
Thousands of security personnel were deployed and roads were blocked by containers.
- "Red alert" -
Protests organized in May by Khan, 70, had degenerated into chaos: the capital had been blocked and clashes had broken out across the country between police and protesters.
The police said that any attempt by PTI supporters to enter Islamabad would be firmly suppressed this time around.
Interior Minister Rana Sanaullah, who Mr Khan says is involved in "the assassination plot" along with current Prime Minister Shehbaz Sharif and a senior military official, issued a "red alert" on Friday, warning of the security threats caused by the gathering.
"The PTI still has time (to cancel)," he said, citing the Pakistani Taliban and al-Qaeda among the extremist groups that could go after Mr Khan.
The government claims that the attempted assassination of the latter was the work of a single man currently in detention. Police have released a "confession" video of the junkyard owner in which he says he acted because Imran Khan was against Islam.
Saturday's rally comes two days after the government named a former spymaster as its next military chief.
The appointment of General Syed Asim Munir has ended months of speculation for a post long considered the real power in the nuclear-armed Islamic nation of 220 million.
Syed Asim Munir served as head of the all-powerful Military Intelligence (ISI) under Khan, but his tenure ended after just eight months.
Pakistan's military, the sixth largest in the world, has considerable influence over the country. It has staged at least three coups since independence in 1947, remaining in power for more than three decades.