Mr Khan’s leadership of nearly four years ended in the early hours of Sunday after a narrow majority of the National Assembly voted for an opposition no-confidence motion.
His defeat means Mr Khan’s return to opposition in the nuclear-armed country of 220 million people.
The former cricketer had clung on for almost a week after thwarting the united opposition’s first attempt to hold the vote.
He dissolved the parliament on April 3 and called for new elections, claiming that the attempt to remove him was a foreign plot.
After the Supreme Court ordered the house to reconvene, parliamentarians from his Pakistan Tehreek-e-Insaf party attempted to delay and block the ballot during a 13-hour assembly session on Saturday.
The vote was only allowed to proceed after the speaker and deputy speaker resigned minutes before midnight, when the Supreme Court was due to sit and potentially hold them in contempt.
On Sunday, Mr Khan spoke publicly for the first time since his defeat. He repeating his charge that he had been thrown out by a foreign plot.
“Pakistan became an independent state in 1947. But the freedom struggle begins again today against a foreign conspiracy of regime change," he told followers on Twitter.
“It is always the people of the country who defend their sovereignty and democracy.”
Nomination papers for the new prime minister were submitted on Sunday. The assembly will reconvene at 11am on Monday to vote for the new prime minister.
Mr Khan’s probable successor is one of the political leaders that he had vowed to purge from Pakistan’s politics when he took power in 2018.
Shehbaz Sharif, 70, is the younger brother of Mr Khan’s predecessor, Nawaz Sharif, and leads the main opposition party, the Pakistan Muslim League (Nawaz), or PML-N.
He lacks the crowd-pulling power of his elder brother but has the reputation as a hard-working, competent and pragmatic administrator after three terms as chief minister in Punjab province.
Born into a wealthy industrialist family, he studied law and spent time in the family business before eventually entering politics.
His tenure in Punjab involved him spending heavily on big-ticket infrastructure items, while his critics said he did too little to tackle the province’s deep-rooted health and agricultural problems.
Mr Sharif faces corruption allegations which his party says are part of a political vendetta by Mr Khan.
In December 2019, Pakistan's anti-corruption watchdog, the National Accountability Bureau, accused him of money-laundering and seized nearly two dozen of his properties. He denies wrongdoing and is currently on bail with a trial still pending.
Mr Sharif is widely believed to have better ties with Pakistan’s powerful military than his more uncompromising brother.
"A new dawn has started... This alliance will rebuild Pakistan," Mr Sharif said in parliament after the vote result was announced.
He said the new government would not slip into the “politics of revenge” by harassing Mr Khan or his supporters.
“I don’t want to go back to the bitterness of the past. We want to forget them and move forward," he said. "We will not take revenge or do injustice; we will not send people to jail for no reason, law and justice will take its course.”
On Sunday afternoon, Mr Sharif submitted nomination papers for the prime ministerial role.
Shah Mahmood Qureshi, Mr Khan’s foreign minister, will run as the PTI party’s candidate.
Both candidates were accepted to run by the National Assembly, which is scheduled to hold a vote at 2pm on Monday.
But Fawad Chaudhry, Mr Khan’s former information minister, said PTI members would resign in protest over the assembly accepting Mr Sharif’s candidacy.
Mr Sharif had been expected in court on Monday as part of the money laundering case against him.
“What can be more insulting for Pakistan that a foreign-selected and foreign-imported government is imposed on it and a person like Shehbaz is made its head,” he said.
Mr Chaudhry called the no-confidence vote a “foreign regime change operation”.
“We think this is a slap on [the face of] people of Pakistan. We reject it. The whole nation expects leadership from Imran Khan and expects from PTI that they’ll come out on streets [against] this foreign conspiracy.”
Mr Khan has called for peaceful protests on Sunday evening.
The World Cup-winning former captain of the national cricket team remains hugely popular among his supporters. Many of them believe his claims that he is the victim of an American meddling because of his determination to pursue friendly ties with China and Russia.
The size of the turnout will give the first indication of how much support he will take into opposition.