Several hundred activists were detained Wednesday as authorities attempted to thwart the protest, which is expected to further weaken a government struggling with political crisis, economic meltdown and Islamist violence.
Security forces are on alert amid fears of violence, and a 19th century British law has been invoked banning protests in Islamabad, Karachi and Lahore, as well as their respective provinces of Sindh and Punjab.
Organisers are hoping that hundreds of thousands of lawyers, opposition supporters and civil activists will join the four-day convoy on the 1,500-kilometre (940-mile) journey to Islamabad, where it is expected to arrive on Monday.
They want unpopular President Asif Ali Zardari to act on his promise to reinstate judges sacked in 2007 by former military ruler Pervez Musharraf, including Supreme Court chief justice Iftikhar Muhammad Chaudhry.
Former Prime Minister Nawaz Sharif, the most popular opposition leader, has joined forces with the lawyers and on Wednesday urged people to rise up to “change the destiny of Pakistan’ in what will be the third such annual march.
Earlier this week, the government threatened charge Sharif with sedition for inciting public rebellion since he was disqualified from contesting elections by the Supreme Court in late February.
Zardari and Sharif have long fought over the future of nuclear-armed Pakistan, a key US ally in the fight against Taliban and Al-Qaeda militancy.
Protests in 2007 ultimately led to Musharraf's resignation and patience is running out with the new civilian government, more than a year after parliamentary elections and six months after Zardari took office.
The authorities braced for possible clashes.
‘We have declared a security alert in the city and taken all possible measures to maintain peace,’ said Karachi city police chief Waseem Ahmed.
In Sindh, thousands of troops were on alert, according to a spokesman for the Rangers unit.
‘Our paramilitary troops are deployed in and around sensitive places in Karachi including government buildings, major hotels, Western food franchises, diplomatic missions and other installations,’ Major Mohammad Ali said.Nevertheless, organisers have vowed to flout the clampdown.
‘A large number of activists from political parties and civil society will join us from dozens of cities in Sindh,’ said Rashid Razvi, president of the Sindh High Court Bar Association.
Sardar Rahim, a spokesman for Sharif's Pakistan Muslim League-Nawaz (PML-N), said party members would join the lawyers outside the Sindh High Court on Thursday.
The PML-N quit the cabinet last year to protest the government’s failure to honour a deadline for reinstating the sacked judges.
The biggest party in the government, the Pakistan People’s Party of Zardari’s assassinated wife Benazir Bhutto, has stalled over disagreements.