- People who looted country will be brought to justice: Imran Khan
- He won the acrimonious vote in parliament for the top seat
- His party was single-largest, but fell short of outright majority
Here are the top 10 updates in this big story:
Imran Khan swore to "bear true faith and allegiance to Pakistan", and to "discharge my duties and perform my functions honestly, to the best of my ability... and always in the interest of the sovereignty, integrity, solidarity, well-being and prosperity of Pakistan" at his oath ceremony.
The former cricketer had invited three of his contemporary players from India, Navjot Singh Sidhu, Sunil Gavaskar and Kapil Dev to his oath ceremony. Only Mr Sidhu made it to Pakistan for the event.
Mr Khan's party had emerged as the single largest party in last month's election but it fell short of outright majority. The PTI has scraped through a simple majority in parliament's confidence vote on Friday on the strength of support from smaller parties. The majority mark was 172; he secured 176 votes.
His only rival, Shahbaz Sharif of the ousted Pakistan Muslim League-Nawaz, or PML-N, won 96 votes. Mr Khan's victory ends the rotating leadership of Pakistan between the PML-N and the Bhutto family-led Pakistan Peoples Party, with periods of military rule in between.
The former cricket legend has struggled for 22 years to turn popular support into electoral gains for the party he launched after retiring from cricket in 1996. Just four years earlier, he had led the Pakistan cricket team to win the 1992 World Cup that turned him into an international celebrity.
But the wafer-thin majority could make it difficult to push through his reforms agenda, particularly given how the opposition which controls Pakistan's Senate, or upper house, accuses him of leaning on the powerful military to rig elections.
The slender majority, and the perception of his links with the military, could make it difficult for him to deliver on cracking down on terror groups, some of them perceived to have the tacit support of the Pakistani security establishment. Action on this front would also dictate, to a large extent, the future of his country's relations with India.
Mr Khan had, after former prime minister Atal Bihari Vajpayee's death, said the two countries could honour the veteran BJP leader's legacy by establishing peace between the two countries.
Prime Minister Narendra Modi had phoned Mr Khan last month and spoken about peace and development in the neighbourhood. Just days earlier, Mr Khan had stressed that he wanted to fix ties between the two countries. "You take one step forward, we will take two," he said.
Many, however, expect Imran Khan to focus on the looming economic crisis. There has been speculation that Pakistan will end up seeking a bailout from the International Monetary Fund or seek support from China.
(With inputs from Agencies)