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.