The election of Mirza was largely a formality after being nominated by the PPP, which has the largest number of seats in the assembly and is preparing to lead the coalition government.Pakistan's challenges
"This is my third tenure in the national assembly and I believe it is time that we all work together to address the challenges facing the country," Mirza said before the vote.
"I am sure that we will be able to face these challenges with the support of parliamentarians, our people and Pakistani media."
The speaker conducts the business of the house, deciding which debates or motions are allowed, and will therefore play a key role in a parliament that is expected to confront Pervez Musharraf, Pakistan's president.
"This is a big test and I hope that with the honour that Allah has bestowed upon you, you will be able to live up to expectations," Chaudhry Pervaiz Elahi, parliamentary leader of the pro-Musharraf Pakistan Muslim League-Q (PML-Q) party, said.
The candidate put forward by the PML-Q won just 70 votes after their crushing defeat in the elections on February 18.Premier nomination
Meanwhile, Bilawal Bhutto Zardari, Bhutto's 19-year-old son and her heir apparent, returned to Pakistan on Wednesday to announce her party's nomination for the post of prime minister, a party spokesman said.
Party insiders say the current front-runner for the position is Yousuf Raza Gilani, who was parliamentary speaker during Bhutto's government from 1993-1996.
They say that the prime minister named by Bilawal may only hold the post for a few months until Asif Ali Zardari, his father, becomes eligible to become premier.
Zardari did not contest the elections because candidacies were filed before his wife's assassination but is likely to fight a by-election in her home constituency in May.