"Who will save Pakistan? Imran Khan, Imran Khan," the crowd chanted.
More than 20,000 people had gathered as the meeting began at 4pm with more on the way, according to witnesses, as the crowd waited for a scheduled appearance by their hero.
Khan's party slogan is "throw this government out and save the country," in a campaign.
The rally, seen as a show of strength, comes two days after Sharif's brother Shahbaz, attracted some 30,000 people at an anti-government protest also in the key political battleground of Lahore.
Pakistan Muslim League-N (PML-N) held on Friday to demand early elections in its political heartland -- it controls the Punjab provincial government despite being in opposition at national level.
Tehreek-e-Insaf Secretary General Arif Ali said Khan "will make important announcements regarding the country's politics and future course of action."
"It is going to be historic. We have our supporters and voters coming from Lahore and its suburban areas and the expected gathering would be over 100,000 people," Malik Zaheer Abbas Khokhar, a member of the party's organising committee said.
"We have been reassured by the provincial police chief about the security of the gathering and smooth flow of traffic for our convoys," Khokhar told AFP.
Senior police official Ghulam Mahmood Dogar told reporters that all steps had been taken to provide "fool-proof" security at the public meeting.
Around 2,000 Pakistanis led by Khan demonstrated in Islamabad on Friday to demand an end to US drone strikes in the tribal areas, claiming they kill more innocent civilians than Taliban and Al-Qaeda leaders.
