Islamabad  - Police detained dozens of protesters in Pakistan's southern port city of Karachi Thursday as hundreds of lawyers and opposition political workers launched a four-day cross-country march for the reinstatement of the country's top judge.

Law-enforcement officers in riot gear bundled protesters into waiting vans after baton-charging a crowd of anti-government activists outside the provincial High Court building in Karachi.

"No one will be allowed to disturb peace in the city," Karachi police chief Waseem Ahmed told reporters shortly after the action.

Authorities have slapped a ban on political rallies and public gatherings in the most populous provinces of Sindh and Punjab, through which the protest rally is to travel before staging a planned indefinite sit-in in the capital, Islamabad, beginning Monday.

More than 500 political activists and lawyers have been arrested in the two provinces over the past two days to ward off mass agitation.

In Quetta, the capital of the south-western province of Balochistan, lawyers led by Ali Ahmed Kurd also started a march, planning to join their comrades in Sindh.

"I'm 1,000-per-cent confident that we will end our sit-in only after the restoration of an independent judiciary," Kurd said.

The demonstrators want Pakistani President Asif Ali Zardari to honour his pledge to restore former Supreme Court chief justice Iftikhar Chaudhry, who was sacked under emergency rule in November 2007 by then-military strongman Pervez Musharraf.

Pakistan's most popular opposition leader and ex-premier, Nawaz Sharif, has thrown his full support behind the lawyers, giving a boost to the protest campaign, which threatens to destabilize the coalition government of Zardari's Pakistan People's Party.

Sharif was recently disqualified by the nation's apex court from holding any public office. His brother, Shahbaz Sharif, also had to step down as Punjab's chief minister after a similar ban.

Sharif accused Zardari of influencing the court's verdict, an allegation repeatedly denied by the president.

The government has warned the former two-time premier of sedition charges for asking the public to participate in the protest, dubbed a "Long March," but Sharif said these were defining moments for the country and "people must come forward to save Pakistan."

The growing political turmoil has raised concerns among Western countries that the showdown would take Pakistan's focus away from its fight against al-Qaeda and Taliban militants plaguing the north-western tribal region near Afghanistan.

US Ambassador Anne W Patterson on Thursday met Sharif at his residence on the outskirts of Lahore, but the details of the talks were not immediately available. (dpa)