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Sharif was speaking via video link from London to a gathering of tens of thousands of people organised by opposition parties to kick-off a countrywide protest campaign in the eastern city of Gujranwala aiming to oust Khan's government.
The former prime minister also accused the chief of Pakistan's Inter Services Intelligence spy agency of being involved in conspiring against his government.
Nine major opposition parties formed a joint platform called the Pakistan Democratic Movement (PDM) last month to begin a nationwide agitation against the government.
Sharif, whose Pakistan Muslim League-Nawaz (PML-N) is the main opposition party, was sacked by the Supreme Court in 2017 on corruption charges and left for London last November for medical treatment.
He blames generals and judges for what he says were trumped up charges.
Pakistan's military denies meddling in politics. The military's public relations wing did not immediately respond to Reuters' request for comment.
Maryam Nawaz, the daughter and political heir of Sharif and Bilawal Bhutto, the son of assassinated former prime minister Benazir Bhutto who heads her Pakistan People's Party (PPP), also addressed the gathering.
Both criticised the Khan government for what they called bad governance and mismanagement of the economy.
The protest campaign comes at a time when Pakistan is experiencing an economic crisis, with inflation touching double digits and negative growth.
Speakers at the gathering called for Khan's resignation and an end to military's interference in politics.
The next general election is scheduled for 2023.
"Go Imran go. Your time is up!" shouted tens of thousands of the opposition supporters gathered at the Gujranwala stadium.
Khan, who came to power on an anti-graft platform and denies the army helped him win, said on Friday he wasn't afraid of the opposition's campaign, which was aimed at blackmailing him to drop corruption cases against their leaders.
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