"President Asif Ali Zardari should resign immediately. He cannot hold free and fair elections because he is holding two offices," Khan told a news conference in the eastern city of Lahore.
His call came after the Supreme Court issued an order for the arrest of the prime minister over corruption allegations, and as 25,000 people took part in the largest political protest in Islamabad since Zardari's government took office.
"The government should immediately announce new elections and should also announce a date for it," Khan told the news conference. "Change is not possible without holding of free and fair elections," he added.