Acting President Sadiq Sanjrani (L) administers the oath to Shahbaz Sharif on Monday night. AFP
The oath was administered by the chairman of Pakistan's Senate, Sadiq Sanjrani, in the absence of President Arif Alvi, a member of Imran's party, who said he was unwell.
The three services chiefs, Pakistan Muslim League-Nawaz (PML-N) Vice President Maryam Nawaz, JUI-F chief Maulana Fazlur Rehman, PPP chairman Bilawal Bhutto-Zardari, and other leaders of political parties were in attendance at the ceremony.
The President’s official Twitter account said he complained of discomfort and has been examined by a doctor.
"President Dr Arif Alvi has complained of discomfort. The physician has examined him thoroughly and has advised him rest for a few days,” the post read.
Earlier today, the National Assembly elected Shahbaz Sharif as the new prime minister while the Pakistan Tehrik-e-Insaf (PTI) boycotted the election process and announced tendering their resignations collectively.
Shahbaz Sharif addresses during the National Assembly session in Islamabad. AP
“Mian Shahbaz Sharif is declared as elected Prime Minister,” Ayaz Sadiq announced following the completion of the counting process.
The PML-N chief, a nominee of the joint opposition parties, received 174 votes for the top post days after Imran was ousted via a no-trust vote by the opposition parties.
Though the opposition alliance had the support of PTI's dissident members, they did not the cast vote and stayed in the opposition’s lobbies.
Shahbaz's election brings to a close a week-long constitutional confrontation that climaxed on Sunday when Imran lost a no-confidence vote, although the nuclear-armed nation is likely to remain prone to political and economic turbulence.
Shahbaz, 70, who has a reputation domestically as an effective administrator more than as a politician, is the younger brother of three-time prime minister Nawaz Sharif.
Analysts say Shahbaz, unlike Nawaz, enjoys amicable relations with Pakistan's military, which traditionally controls foreign and defence policy in the country of 220 million people.
After the vote, Sharif vowed to tackle an economic malaise that has seen the rupee hit an all-time low and the central bank hike rates by its largest amount in decades last week.
"If we have to save the sinking boat, what we all need is hard work, and unity, unity and unity," he said in his maiden speech to parliament. "We are beginning a new era of development today."
Just minutes before the vote, legislators from Imran Khan's party resigned en masse from the lower house of parliament in protest at the expected formation of a government by his political foes.
"We are announcing we are all resigning," Shah Mahmood Qureshi, former foreign minister and vice president of Imran's party, told the assembly. The mass resignations will require fresh by-elections in well over 100 seats.
Imran's Pakistan Tehrik-e-Insaf (PTI) party had submitted papers nominating Qureshi as its candidate for prime minister.
The younger Sharif emerged as the leader of a united opposition to topple Imran Khan, a former cricket star who has claimed that the United States was behind his downfall, which Washington has denied.
Shahbaz said in an interview last week good relations with the United States were critical for Pakistan for better or for worse, in stark contrast to Imran's prickly ties to Washington.
Nawaz Sharif was barred by the Supreme Court in 2017 from holding public office and subsequently went abroad for medical treatment after serving just a few months of a 10-year jail sentence for corruption charges.
"There can't be any bigger insult to this country," Imran, ousted in a no-confidence vote by the same assembly in the early hours of Sunday, told reporters on Monday on the prospect of Shahbaz being elected.
No elected prime minister has completed a full term in the nuclear-armed nation since it won independence from colonial power Great Britain in 1947, though Imran is the first to be removed by a no-confidence vote.
The military has ruled the country of 220 million people for almost half its nearly 75-year history. It viewed Imran and his conservative agenda favourably when he won election in 2018.
But that support waned after a falling-out over the appointment of military intelligence chief and economic troubles that last week led to the largest interest rate rise in decades.
Khan remained defiant following his defeat in parliament. Thousands of his supporters in several cities held protests against his ousting that went on until Monday's early hours.
“The NSC, chaired by Prime Minister Shahbaz Sharif, discussed the telegram received from the Pakistan Embassy in Washington. Pakistan’s former ambassador to the US Asad Majeed briefed the committee on the context and content of his telegram..."
Candidates for Pakistan's next prime minister are due to file nomination papers on Sunday after incumbent Imran Khan lost a no-confidence vote in parliament, bringing an end to the former cricket star's nearly four years in power.
Pakistan's cabinet has given its nod to a law proposed by the Cabinet Committee on the Inter-Governmental Commercial Transactions to sell stakes of oil and gas companies and government-owned power plants to different countries.
The UN climate summit clinched an early victory on Thursday, with delegates adopting a new fund to help poor nations cope with costly climate disasters.
Dr Sheikh Sultan listened to a detailed explanation from Sultan Al Neyadi and Hazzaa Al Mansoori, about the longest space mission in Arab history, which Al Neyadi underwent and extended to 6 months aboard the ISS.
The campaign, initiated by the UAE, seeks to provide relief to Palestinian brothers in the Gaza Strip by offering urgent assistance.