Former prime minister Nawaz Sharif quit Pakistan's ruling coalition on Monday over differences on the reinstatement of judges sacked by former president Pervez Musharraf and on who should replace him.

Sharif, head of the second-largest party in the coalition, had laid down a Monday deadline for restoration of the judges sacked by Musharraf, who resigned last week to avoid the coalition's plans to impeach him.

Sharif said his Pakistan Muslim League-N (PML-N) had decided to sit in opposition after Asif Ali Zardari, leader of the Pakistan People's Party, failed to honour his commitments.

"We have taken this decision after we failed to find any ray of hope and none of the commitments made to us were fulfilled" by Zardari, he told a news conference.

"This situation forced us to withdraw our support and sit on opposition benches," he added. However, he vowed to "continue struggle for restoration of judges and genuine democracy in Pakistan."

Sharif also announced that his party had decided to put up its own candidate against Zardari who has been nominated for the presidential slot by the PPP.

He named retired supreme court chief justice Saeed uz Zaman Siddiqui as his party's candidate in the polls scheduled for September 6.

"We have requested Saeed uz Zaman Siddiqui to accept our offer to become presidential candidate," Sharif said after a crucial meeting of his party. "He is a good Pakistani who is a non-partisan person," he added.

Sharif accused Zardari of repeatedly breaking promises over the reinstatement of some 60 judges sacked by Musharraf under emergency rule in November last year.

"The PPP even nominated Zardari for president and announced the election schedule without consulting our party," he said. "The PPP also violated the latest agreement that the judges will be restored 24 hours after the resignation of Musharraf."