Critics of President Asif Zardari say the amnesty is unacceptable
Pakistan's Supreme Court has begun hearing challenges to a controversial law granting senior politicians and others amnesty from corruption charges.
The law lapsed last week after failing to win parliamentary approval. If it is ruled invalid, President Asif Zardari could face challenges to his rule.
Ex-President Pervez Musharraf brought in the amnesty by decree in 2007.
It was aimed towards a possible power-sharing deal with Mr Zardari's late wife, Benazir Bhutto.
She returned to Pakistan from abroad after the so-called National Reconciliation Ordinance (NRO) was signed into law, but was assassinated soon after.
Mr Zardari could face challenges to his rule
There are still court cases pending in Pakistan against Mr Zardari, who spent years in jail after being indicted for corruption, charges he says were politically motivated.
His office currently provides him with immunity from prosecution.
After his wife's death, Mr Zardari won elections in 2008 that led to him replacing Mr Musharraf as president.
The BBC's Syed Shoaib Hasan says Mr Zardari's popularity has dipped and there are increasing question marks about his ability to lead.
Our correspondent says any court decision against the law is likely to increase pressure on Mr Zardari and his Pakistan People's Party for him to step down to face the charges.
Critics of the amnesty say it is illegal to help so many politicians and officials to escape prosecution for alleged wrongdoing.
In November, the PPP failed to win parliamentary approval for the amnesty, as demanded by the Supreme Court.
"We are aware that none of the members of parliament are in favour of the NRO," Chief Justice Iftikhar Chaudhry told the court on Monday.
"The Supreme Court has already ruled that the parliament cannot pass any law that inhibits or prevents any common citizen from receiving justice."
Pakistan's attorney-general, Shah Khawar, told the court that the government would not defend the ordinance and would abide by any decision of the Supreme Court.
The court asked the government to provide a complete list of all those people who had benefited from the law.
Legal observers say the process could take a while but the judges are likely to rule that the amnesty is constitutionally invalid.
Our correspondent says this could then open the way to attempts to impeach Mr Zardari or to challenge his immunity from prosecution.