KARACHI, Pakistan (CNN) -- Police in Pakistan arrested lawyers holding a public protest to demand that the government immediately restore judges the previous president had ousted, Karachi's police chief told CNN Thursday.



Lawyers shout slogans Thursday in Karachi on the eve of a march to Islamabad.



Among those detained were Muira Malik, former president of the Supreme Court Bar Association and leader of the Karachi lawyers movement, and retired high court justice Rasheed Razvi, police Chief Wasim Ahmed said.



The lawyers were arrested under an order that outlaws public gatherings.



The police chief also said authorities had credible information that there was a terror threat against the march and he had warned the lawyers before they started that it would be a public danger.



Up to 500 lawyers in the Karachi group planned to join thousands of other demonstrators heading to the capital, Islamabad, as part of a four-day "Long March."



The demonstrators plan a massive sit-in at the parliament building Monday.



"Our movement is a peaceful movement," organizer Razvi, president of the Sindh High Court Bar Association, said before his arrest. "When we protested the last time, there were hundreds of thousands of people and not one grass was broken, not one leaf was broken."



The demonstrators began walking from the gates of the Sindh High Court, heading to a dozen buses that waited to ferry them to their next stop: the city of Hyderabad.



"The rule of baton and bullets cannot last," the protesters chanted.



They want President Asif Ali Zardari to live up to a promise to reinstate judges sacked by then-President Pervez Musharraf. Among the dismissed judges was the chief justice of the Supreme Court, Iftikhar Muhammad Chaudhry.



After sweeping into power in parliamentary elections last year, Zardari's Pakistan Peoples Party promised to reinstate the judges within 30 days of taking office. The deadline came and went.



The government responded to the recent intensification of protests by banning political demonstrations in two of the country's biggest provinces -- Punjab and Sindh. It also detained several hundred activists Wednesday.



But the protesters said they will not be deterred. Their movement now, they said, isn't so much about reinstating Chaudhry as it is about restoring the office of the chief justice.



"We will start the long march from the province as we have promised the nation," Razvi said.



The country's largest opposition party, the Pakistan Muslim League-N (PML-N), is backing the lawyers -- but for reasons of its own.



Party head Nawaz Sharif accuses Zardari of being behind a February Supreme Court decision that bars Sharif from holding public office.



The court also stripped Sharif's brother, Shahbaz, of his post as chief minister of Punjab -- the Sharif party's power center.



Supporters of PML-N have responded by holding massive rallies, some of which have turned violent in recent days.



CNN's Thomas Evans and Zein Basravi contributed to this report
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question: What did the protesters want?, answer: President Asif Ali Zardari to live up to a promise to reinstate judges sacked by then-President Pervez Musharraf. | question: Who has vowed to reinstate them?, answer: President Asif Ali Zardari | question: What will happen after the election?, answer: Zardari's Pakistan Peoples Party promised to reinstate the judges within 30 days of taking office. | question: What did the president's party vow?, answer: to reinstate the judges within 30 days of taking office. | question: Who was arrested?, answer: lawyers | question: Who was arrested?, answer: lawyers | question: Where is the capital?, answer: Islamabad, | question: What do protestors want?, answer: the government immediately restore judges the previous president had ousted, | question: Who vowed to reinstate judges?, answer: Zardari's Pakistan Peoples Party | question: Where was the protest?, answer: KARACHI, Pakistan | question: Who is heading to Islamabad?, answer: Lawyers | question: Whose party vowed to reinstate judges?, answer: Zardari's Pakistan Peoples | question: Who did they want restored to office?, answer: judges the previous president had ousted, | question: Who was arrested before the protest?, answer: lawyers