ISLAMABAD: Pakistan's main opposition party on Thursday called on President Asif Ali Zardari to resign after a court threw out an amnesty protecting him and senior government figures from corruption charges.

The Supreme Court declared Wednesday that a 2007 National Reconciliation Ordinance (NRO) which contained the amnesty was unconstitutional and struck it down, paving the way for thousands of criminal cases to be revived.

Zardari is immune from prosecution while in office, but that immunity and his eligibility for president could now be challenged, as a number of graft cases were pending against him when the NRO was passed.

"We are suggesting President Zardari should resign on moral grounds. All the cabinet members must immediately tender their resignations," said Siddiqul Farooq, spokesman for the opposition Pakistan Muslim League-N (PML-N).

"President Asif Ali Zardari should resign on moral grounds and should not depend upon the crutches of the constitution," he added.

Pakistan's constitution guarantees Zardari immunity, but also states that presidential candidates must be pious, honest and truthful and never have been convicted in a criminal case.

Although Zardari has spent years in jail over corruption charges, he alleges the charges were politically-motivated and questions hang over whether he was ever actually convicted of any crime.

The NRO was passed in October 2007 by then-president Pervez Musharraf, under pressure to hold elections and end about eight years of military rule.

It quashed charges against a number of politicians including Zardari and his wife and ex-prime minister Benazir Bhutto -- who was assassinated two months later -- to allow them to stand for office.

Zardari's Pakistan People's Party (PPP) went on to win elections in 2008, restoring civilian rule, but the NRO expired at the end of last month and the PPP did not have enough support to renew the ordinance in parliament.

Other NRO beneficiaries include the interior and defence ministers.

Senior figures in the PML-N, led by former prime minister Nawaz Sharif, have already called on Zardari to give up wide-reaching powers inherited from Musharraf to sack the prime minister and dissolve parliament.

Analysts say such a move may be the only way to save his struggling presidency as he faces rock-bottom public approval ratings and strained relations with the powerful military.

Any political fracas will likely unnerve Islamabad's Western allies, who want stability to allow Pakistan to focus on quashing Islamist extremism.