By Tim Shipman and Massoud Ansari in Karachi.
March 25, 2007.

Two exiled former prime ministers of Pakistan will launch a joint attempt this week to drive the current president, Pervez Musharraf, from power.

Benazir Bhutto and Nawaz Sharif agreed to join forces during a meeting in London after weeks of political crisis in Pakistan have left its military strongman with a tenuous grip on power.

Thousands of their supporters are expected to take to the streets tomorrow in co-ordinated demonstrations, culminating in a rally in the capital, Islamabad.

Miss Bhutto's Pakistan Peoples' Party and Mr Sharif's Pakistan Muslim League will protest against Gen Musharraf's decision to sack the country's chief justice, who had opposed his attempts to cling to power. But last night a close ally of Miss Bhutto, who has twice been prime minister, made clear that the removal of Gen Musharraf, who is also army chief of staff, is the -ultimate goal.

Wajid Shams-ul-Hasan, the former Pakistani high commissioner in Britain, said: "The seriousness of the crisis in Pakistan means that we have formulated a joint strategy to neutralise Gen Musharraf and to ensure that the next elections are free and fair. This has become a very explosive situation for Musharraf. He should go and the army should go back to barracks."

spokesman for Mr Sharif said: "We want his resignation and then free and fair elections without Musharraf. With him sitting there, you cannot get free elections."

The current crisis arose because Gen Musharraf wants Pakistan's National Assembly to rubber stamp his rule for another five years before it is dissolved for elections, due later this year.

The chief justice, Iftikhar Chaudhry, had said the plans were unconstitutional and pressed for Gen Musharraf to surrender his army post as well. Gen Musharraf -suspended him on March 9 claiming the judge had abused his position, provoking the worst crisis since he overthrew Mr Sharif and seized power in a military coup in 1999.

The pact between Mr Sharif and Miss Bhutto is significant since, until now, the public protests over the crisis have been led by lawyers rather than politicians.

Half a dozen judges, including a high court judge and deputy attorney general, have tendered their resignations and a further 16 are expected to stand down in protest.

On Friday, lawyers again took to the streets brandishing banners reading: "It is death for Musharraf".

If they return to Pakistan, Miss Bhutto and Mr Sharif face arrest on corruption charges, which they each claim are politically motivated. They are still the de facto leaders of their political parties.

After two hours of talks last week, Mr Sharif said: "We have jointly decided to struggle against this military dictatorship and do everything within our means to stop the brutalities Musharraf is committing against institutions in Pakistan."

He met Miss Bhutto after rumours that she had been in talks with Gen Musharraf. Mr Hasan, Miss Bhutto's ally, accused Gen Musharraf's supporters of spreading the rumours to divide the opposition. "There are no differences between us," he said.

Mr Hasan confirmed that Miss Bhutto was prepared to risk imprisonment to return to Pakistan and run for election, either to the National Assembly or to the Senate. "The time has come for her to go back," he said. "Musharraf may dare to arrest her but we are confident that she will be cleared by the judiciary. Musharraf has no moral authority."

The next flashpoint for Gen Musharraf will come when the supreme judicial council considers the fate of the chief justice.

Imran Khan, the former Pakistan cricketer, who now leads his own Movement For Justice party, said that Gen Musharraf was in a lose-lose situation. "If he gets the decision against Mr Chaudhry, nobody is going to accept it and these protests will increase tenfold. If Mr Chaudhry is reinstated as a chief justice, we now can see that Musharraf won't be able to continue as a president and army chief of staff at the same time." He added: "Once he is out as chief of staff, he will become irrelevant."

Legal experts say that revulsion at the treatment of the chief justice has backfired on the president. Syyeda Abida Hussain, a former Pakistan ambassador in Washington, said: "The chief justice of Pakistan suddenly taking a stand against a military dictator has motivated all of us. It is really something from God." Another lawyer claimed: "It seems like a beginning of the end for Musharraf."

Gen Musharraf's position has been further imperilled by an apparent change of heart in the US, where diplomats and intelligence officials are disenchanted with the president's failure to combat fully fundamentalist Muslim terrorists operating in the tribal areas in the north-west of the country who have repeatedly crossed into Afghanistan to attack British and American forces.

A CIA report leaked to the New York Times a week ago revealed that the Bush administration would be content to see Gen Musharraf replaced by his army deputy, Ahsan Saleem Hyat, and the former banker Mohammedmian Soomro installed as president. It concluded that a takeover of Pakistan by extremists Islamic mullahs - the doomsday scenario long feared by Washington - was no longer the most likely outcome of his removal from power.

A British diplomatic source stressed that while Britain is happy to do business with Gen Musharraf as long as he continues counter-terrorist co-operation, the loyalty of the British government is to Pakistan's "institutions rather than individuals".

Miss Bhutto called on the international community to force Gen Musharraf's hand. "If democracy must be defended in Afghanistan, then democracy in Pakistan must be defended, too," she said. "It is important for the international community to stop turning a blind eye."