Pakistan's president faced fresh calls to step down Thursday after the Supreme Court struck down an amnesty that had protected the increasingly unpopular leader and several of his political allies from corruption charges.

The decision late Wednesday sharpened political tensions in the nuclear-armed nation just as the United States and its other Western allies want it to unite and fight against Al Qaeda and Taliban militants based along the Afghan border.

While it is generally agreed that President Asif Ali Zardari has immunity from prosecution as president, his opponents now plan to challenge his eligibility to hold the post. Zardari and his aides say any corruption charges against him are politically motivated and that he will not step down.

Critics said he was morally obligated to resign, at least while the court heard any challenges to his rule.

"It will be in his own interest, it will be in the interest of his party and it will be good for the system," said Khawaja Asif, a senior leader from the opposition Pakistan Muslim League party.

The amnesty was part of a U.S.-brokered deal with former military ruler Pervez Musharraf that allowed former Prime Minister Benazir Bhutto to return home from self-exile and participate in politics without facing charges her party says were politically motivated. Zardari, Bhutto's husband, took control of the party after Bhutto was assassinated in 2007.

The amnesty, known as the National Reconciliation Ordinance, either stopped corruption investigations or probes into other alleged misdeeds or wiped away convictions in cases involving up to 8,000 ministers, bureaucrats or politicians from across the spectrum.

Civil rights activists have long argued that the amnesty unfairly protected the wealthy elite.

Zardari has long been haunted by corruption allegations dating back to governments led in the 1990s by his late wife. He spent several years in prison under previous administrations. The Supreme Court this week heard allegations he misappropriated as much as $1.5 billion.

The court on Wednesday singled out an alleged multimillion dollar money laundering case involving Zardari and his late wife that had been heard in a Swiss court until the attorney general under Musharraf withdrew proceedings against them last year as a result of the amnesty.

The court said this was illegal and ordered the government to ask Swiss authorities to reopen the case.

Pakistani papers welcomed the decision as a victory for justice. Many editorialists said it boded ill for Zardari.

"Zardari: an accused president," read the headline over a front-page story by a well-known critic in The News.

Pakistani political analyst Rasul Bakhsh Rais doubted that Cabinet ministers and other politicians affected by the ruling would simply step down. He noted that investigative and prosecuting entities in Pakistan are not really independent of the government.

"They will play all these tricks and they will stay in power," Rais said, predicting many messy court battles ahead.

Some analysts said Zardari may be able to take some of the sting out of his opponents attacks — and ultimately survive in office — if he gives up many of the powers he inherited from Musharraf.

A few weeks ago, amid mounting pressure, Zardari relinquished command of the country's nuclear arsenal and said he would give up more powers soon. But that's a promise he's made before, including in a major speech to lawmakers just days after being sworn in.