ISLAMABAD - Pakistan's top court yesterday struck down an amnesty that had protected the president from corruption charges, paving the way for challenges to his shaky rule just as the U.S. wants Islamabad to step up its fight against Islamist militants.

The main opposition party immediately called on President Asif Ali Zardari to resign, adding to the political turmoil in this nuclear-armed nation at a time of surging violence by the Taliban and al Qaeda. The U.S-backed president already is deeply unpopular and under pressure to give up much of his power.

The decision also leaves thousands of other officials, including Cabinet ministers and bureaucrats loyal to Zardari who had also been shielded by the amnesty, vulnerable to reopened corruption and other criminal cases.

"All the benefits given under the [amnesty] - cases withdrawn, acquittals made - are declared void," Chief Justice Iftikhar Mohammed Chaudhry said in announcing the ruling of the 17-member bench.

Presidential aides said the government would issue a formal response after thoroughly reviewing the judgment. They pointed out that while he remains president, Zardari has immunity from prosecution.

The main opposition party, the Pakistan Muslim League-N, said Zardari should step down, though it insisted it was not agitating for midterm elections nearly two years after Zardari's party won the majority in parliament.

"After the court ruling, Asif Ali Zardari should morally step down," said Sadiqul Farooq, a PML-N spokesman.

Zardari aides scoffed at the notion.

"The president of Pakistan has no intention of stepping down from his office at this point or anytime in the future," spokeswomen Farahnaz Ispahani said. "There is no constitutional or political reason for the president to resign."

The amnesty was part of a U.S-brokered deal with former military ruler Pervez Musharraf that paved the way for former Prime Minister Benazir Bhutto to return home from self-exile and take part in politics without facing cases that 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, had angered some civil rights activists and ordinary Pakistanis who said it protected the wealthy elite who govern the impoverished, corruption-plagued nation from being punished for their alleged crimes.