Pakistan's top court struck down an amnesty Dec. 16 that had protected U.S.-allied President Asif Ali Zardari from corruption charges.

The ruling is a major blow to the desperately unpopular Zardari and could mark the beginning of his downfall, analysts said.

While he enjoys immunity from prosecution as long as he remains president, Zardari's opponents plan to challenge his eligibility to hold the post.

The court's decision also left thousands of others who had been shielded by the amnesty vulnerable to reopened corruption and other criminal cases. Interior Minister Rehman Malik, the country's top civilian security officer, is among those at risk of prosecution.

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 participate in politics without facing charges that her party says were politically motivated. Zardari, Bhutto's husband, took control of the party after Bhutto was assassinated in 2007.