Supporters of Khan clashed with police overnight as he remained holed up in his Lahore residence, defying attempts to arrest him.
Police detained at least two dozen supporters of the former premier ahead of the planned rally, which was to start in Lahore's upscale Zaman Park area where Imran Khan lives.
The country is rebuilding from the summer’s catastrophic flood that killed nearly 1,739 people.
The country's election commission said the former premier would not be able to hold public office for five years.
Tens of thousands of displaced people are in urgent need of aid– but relief workers say support so far has been inadequate.
The former prime minister mentioned the alleged plot against him in a speech in the city of Sialkot and claimed he had recorded a video message detailing it all.
Huge rallies held in cities across the South Asian country in favour of ex-premier and against "foreign interference", a day after Khan's removal in a parliamentary no-trust vote.
The opposition rejected the move, saying the policy is an attempt to rig the next elections.
The Supreme Court agreed on Tuesday to release jailed former prime minister Nawaz Sharif on bail for six weeks to receive medical treatment but said he would not be allowed to leave the country.
The younger brother of ousted premier Nawaz Sharif launched the election campaign for his Pakistan Muslim League-N from the country's largest city, Karachi, after his brother's ban from holding public office.
The death of 8-year old Zainab Ansari's has sparked outrage and protests, prompting Pinjab's chief minister to fire the police chief.
A long-time loyalist of Nawaz Sharif, the former prime minister who was ousted by the Supreme Court last week, Shahid Khaqan Abbasi will act as a placeholder for Sharif's younger brother Shahbaz.
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