"After having sustained our sit-in in Islamabad for more than a month, the party leadership believes now the time has come to spread this movement all over the country," a PTI official said on Wednesday.
PTI has a strong presence in Karachi, where workers have been regularly organising rallies since the beginning of the Azadi march Aug 14.
Earlier, Imran Khan announced that he would go to Karachi to thank party workers only after securing Prime Minister Nawaz Sharif's resignation.
Party wants to spread the movement all over the country: PTI
"The decision to expand the movement is in response to the government's stubbornness. If the prime minister believes he can beat us with the support of two houses of parliament, he is either misreading the situation or his advisers are misleading him," a senior PTI leader said.
PTI's deputy general secretary Imran Ismail has been tasked with making arrangements for the Karachi rally.
The protests led by PTI chief Imran Khan and Pakistan Awami Tehreek chairman Tahirul Qadri began Aug 15 in an attempt to overthrow Prime Minister Nawaz Sharif, whom they have accused of rigging the 2013 general election.