Mulk replaces Justice Tassaduq Hussain Jillani who had resigned from his post of CEC yesterday, after being appointed as the next Supreme Court Chief Justice.
The oath was administered to the new CEC by outgoing Chief Justice Iftikhar Chaudhry.
On November 27, Prime Minister Nawaz Sharif had given the go-ahead for Jillani to be appointed as the next Chief Justice as Chaudhry is set to retire from the post on December 12.
Meanwhile, Chief Justice Chaudhry said it was the judiciary which had ensured that for the first time in the country's history, state institutions were functioning within their constitutional perimeters.
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He emphasised that the judiciary would ensure the supremacy of the law and the Constitution at all costs.
"Speedy justice is the right of every citizen," said the country's top judge.
The Chief Justice said the judiciary would in a few days set its goals for the future and added that because of its decisions, its dignity has risen among the Pakistani public as well as the world over.