Thousands of black-suited lawyers gathered in Karachi at the start of the "long march" in a motor convoy that will slowly cross the country, and finish outside parliament in Islamabad on Thursday.
The lawyers want the government to immediately restore about 45 judges sacked by Musharraf last November, including his bitter rival, the former chief justice Iftikhar Muhammad Chaudhry.
"Today is the historic day that the lawyers and judges have come out to protect the country and the constitution," said Sabihuddin Ahmed, the deposed head of the Sindh high court, waving off the procession in Karachi.
In a reprise of last year's lawyers' movement which triggered Musharraf's dramatic political slide, protesters chanted "Go, Musharraf, go!" and burned an effigy of the president.
Lawyers' leader Aitzaz Ahsan promised the march would remain "peaceful until the end", but the possibility remained of a violent clash with security forces.
The protests are a potent challenge to both Musharraf and the fragile government led by the party of the assassinated politician Benazir Bhutto. Supporters include human rights activists, retired army generals and religious and rightwing political parties.
Musharraf, though, says he is not fazed. In a press briefing on Saturday he appeared relaxed, vowing never to allow his presidency to be diminished to the extent that he would become a "useless vegetable".
The president still enjoys the powerful support of the army and America. Amid a flurry of speculation that Musharraf was going to resign, President George Bush recently phoned to reiterate his support.
The so-called long march is also a major headache for the ruling Pakistan People's party (PPP), which has found itself trapped between American policy and intense domestic pressure for Musharraf's dismissal.
The PPP has tried to ease Musharraf out of office, proposing a package of measures under which his powers would be cut. But they have been out-manoeuvred by coalition partner Nawaz Sharif, who appears to have captured the public mood with demands that all judges should be restored and Musharraf tried for treason.
Today the PPP moved its policy in line with Sharif's, saying it agreed that Musharraf should be impeached and prosecuted. "The man must be held accountable," said spokesman Farhatullah Babar.
The protest comes as the government struggles to stop Islamist violence and to slow a rapid economic slide.
The march will be physically difficult, with summer temperatures in many areas already soaring above 40C. The march was due to pass through Multan tomorrow and Lahore on Wednesday.
The lawyers' leaders say they will mount a sit-in outside parliament until their demands are met, but remain vague about how far they will go in confronting the authorities.
There had been speculation that lawyers would also march on Musharraf's residence in Rawalpindi, but senior lawyer Athar Minallah ruled it out. "There is no need for that because it appears as if president and parliament are together," he said.
The army spokesman, Major General Athar Abbas, said he did not expect that the army would intervene. "It is expected that the interior ministry will ensure that the law and order situation won't get out of control," he said.
© 2008 The Guardian