"I urge all Pakistanis not to join the long march as we have credible information that enemies of Pakistan could take advantage of the situation," Rehman Malik, the interior ministry chief, said.
"The long march cannot be allowed on Constitution Avenue, but lawyers can hold their protest at any alternate venue outside Islamabad, so that life here is not disrupted."
Police have temporarily detained scores of activists across the country, including five people among hundreds of lawyers and Sharif supporters who defied a ban on demonstrations in the central city of Multan on Saturday.
Al Jazeera's Imran Khan, reporting from the city of Lahore, said: "We have been told that people are getting to Islamabad, they are going by rail and by car.
"They are taking off their uniforms, they are not going in their distinctive black outfits to try and break through the cordons."
The lawyers are protesting against the refusal of Asif Ali Zardari, the president, to reinstate the senior supreme court judge and a number of other judges dismissed by Pervez Musharraf, his predecessor.
The opposition Pakistan Muslim League party of Nawaz Sharif has backed the lawyers' march on Islamabad after the court barred him and his brother from elected office.
"Most analysts here will tell you ... Zardari doesn't want the chief justice reinstated because he feels he will reopen corruption cases against him," Khan said.
"Nawaz Sharif, on the other hand, wants him to be reinstated because he feels that if he supports him he will overturn the ban." Minister resigns
The resignation of Sherry Rehman, Pakistan's information minister, on Saturday added to the government's woes.
Rehman had been a key adviser to Zardari, reportedly stepped down after differences with other government officials over media restrictions.
Pakistan's The News
reported that Rehman resigned "in protest" after she failed to convince Zardari to lift a ban on private Geo television.
The channel said its cable transmission had been blocked on Zardari's orders because of its coverage of the protests.
Zardari's office dismissed the claim.