Authorities placed 369 people under "preventive detention" last week to keep them from campaigning for a boycott of upcoming national elections, a government spokesman said.
Spokesman Younis M. Sethi said the dissidents were detained under martial law regulations because "they were trying to create a law-and-order situation and trying to persuade the people not to take part in the elections."
Sethi said the government wants to be sure that next week's national and provincial assembly elections "are held in a peaceful atmosphere."
Opposition sources claimed that 1,500 people have been arrested in Sind province, but Sethi said the figure is "highly exaggerated."
Newspaper reports said police are holding more than 650 people until after National Assembly elections Monday and elections for provincial assemblies Thursday.