Pakistan President Mamnoon Hussain administered the oath at a ceremony in the capital, Islamabad, on Saturday where political and military leaders, former sports stars and diplomats participated.
Khan was elected as the prime minister by parliament on Friday as his party, the Pakistan Tehreek-e-Insaaf (PTI), swept the election, vowing to bring an era of accountability and prosperity to the country.
PTI won the most seats in the July 25 election but fell short of an outright majority. It allied with independents to form a coalition.
In the national assembly, Khan secured 176 votes on Friday, beating Shehbaz Sharif, president of the Pakistan Muslim League-Nawaz (PML-N), who got 96.
“In this country, before everything else, we have to make sure there is accountability,” said Khan, in a speech after the vote in parliament.
“Those who have looted this country and indebted it, I promise this today that no one will escape.”
The long-time opposition politician takes the reins of power for the first time in his 22-year political career, having led a strenuous campaign against corruption and government mismanagement since he retired as one of Pakistan’s most popular cricketer in 1992.
The PTI’s election victory was marred by widespread allegations of irregularities in the vote counting process, and opposition parties on Friday decried the results as “unacceptable”.
In his speech, Khan welcomed any investigation into the election results, saying he would support the process. He also committed to empowering parliament, and attending at least twice a month to answer other lawmakers’ questions.
Al Jazeera’s Kamal Hyder, reporting from Islamabad, said Pakistan’s new leader will need to overcome strong opposition in parliament.
“It is going to be a difficult task for Imran when he starts…he’ll have an opposition that will be strong by the looks of it and that may entail problems when it comes to legislation,” correspondent said.
“He has many challenges on the internal front and the external front (but) Imran Khan is adamant that he will deliver.”
SOURCE: AL JAZEERA AND AGENCIES