Pakistan's Attorney General has resigned, accusing the government of preventing him carrying out Supreme Court orders to reopen old graft investigations into President Asif Ali Zardari.

Anwar Mansoor's announcement was the latest chapter in a simmering dispute between the judiciary and Zardari that risks de-stabilising the government just as Washington wants it to focus on the threat posed by al-Qaeda and Taliban militants close to the Afghan border.

The Supreme Court last year struck down a controversial amnesty (NRO) protecting Zardari and scores of other top officials from prosecution over allegations of corruption dating back several years. Earlier this week, it ordered those cases reopened.

Anwar Mansoor said the Law Ministry had been denying him access to documents needed to carry out the Supreme Court order.

"The Supreme Court is our top institution, and there is no way you can defy its orders," Anwar Mansoor said after announcing his resignation on the steps of the building in the capital, Islamabad.

President Zardari is a major beneficiary of the graft amnesty, part of a broader US-backed deal (NRO) to allow his wife and her political allies to return to Pakistan in 2007 and take part in elections safe from prosecution on charges they maintained were politically motivated.

His supporters and many independent experts say President Zardari has immunity from prosecution so long as he is president.

The Supreme Court has zeroed in on one case taken up by the Swiss government against President Zardari that was halted in 2008 under the amnesty. President Zardari and Miss Bhutto were found guilty in absentia in a Geneva court in 2003 of laundering millions of Swiss francs (dollars). They were handed six-month sentences and fined, but both punishments were automatically suspended when they appealed.

On Wednesday, the Supreme Court ordered the government to contact Swiss authorities to reopen the case.

Swiss prosecutors told reporters this was impossible because President Zardari has immunity.

The fireworks in the court this week came as the country's lawmakers agreed on the draft of a constitutional reform package that would strip the presidency of the power to fire an elected government and reduce it to a more ceremonial role.

The opposition had criticised President Zardari for being too slow to relinquish powers accumulated during the rule of his predecessor, Pervez Musharraf, the army chief who took over the government in a 1999 coup. President Zardari, used to being on the receiving end of criticism in the media, has been praised for giving up his powers.

The draft was introduced in parliament Friday. It has the support of all major political parties and is expected get the approval of the house when it is put to the vote, the date of which is unknown.

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