Pakistan's main opposition party has called on Asif Ali Zardari, the country's president, to relinquish much of his powers.

The latest call on Sunday from Shahbaz Sharif, the brother of main opposition leader Nawaz Sharif, comes amid growing clamour for Zardari to assume a ceremonial role or resign.

Zardari's position as president has been weakened by the expiration of an amnesty protecting him and several key allies from graft prosecution, raising the possibility of legal challenges to his rule in the coming months.

The Pakistani president has promised to give much of his powers to the prime minister, in line with Pakistan's original constitution, but has been accused of foot-dragging by an angry opposition.

Zardari transferred command of the country's nuclear arsenal to Yousef Raza Gilani, the prime minister, on Friday and promised to surrender other key powers by the end of 2009, including the authority to fire an elected government and appoint top military chiefs.

'Abolish amendment'

Shahbaz, who is chief minister of Pakistan's largest province, Punjab, said Zardari must act now on promises he made earlier on abolishing the measure known as the 17th constitutional amendment.

"I would ask the president to immediately abolish the 17th amendment," Sharif said to reporters in the Pakistani city of Lahore.

"The nation would appreciate this act."

Zaradari's deepening unpopularity has put Washington in a bind over its avowed commitment to political stability in Pakistan.

If Zardari is forced from power, either on corruption charges or through a collapse of his ruling party, the US might have to deal with new leaders who are no better able to solve the country's problems.

However, a military coup to oust Zardari appears unlikely, as does impeachment since he heads the largest party in parliament.

The upheaval comes as the administration of Barack Obama, the US president, is expected to announce a new strategy this week for defeating the Taliban in neighbouring Afghanistan and on Pakistan's northwestern border.

Zardari inherited sweeping presidential powers from General Pervez Musharraf, who seized power after a 1999 military coup and resigned last year.