The "Walk for Democracy," scheduled for Saturday, was organized by a coalition known in Malaysia as Bersih 2.0, using the Malay word for clean, The Guardian reported. The government declared the group illegal last week.
Organizers expect as many as 300,000 people. But police have denied Bersih a permit and, in exercises this week, soldiers practiced holding up signs that said "Disperse or we will shoot."
By Friday, more than 200 activists had been detained. Even the wearing of yellow, chosen as the color of the protest movement, was declared a crime.
Aliran Monthly, a publication backing the protest, posted a statement on its Web site that said "Kuala Lumpur is under siege."
"Ninety-one Malaysians have been barred from entering the city centre under threat of arrest," Aliran said. "A court order has been obtained to enforce this ruling."
Bersih is calling for fair elections, which must take place by 2013. Barisan Nasional, or the National Front, has ruled Malaysia since 1955, but the party lost a significant number of seats in 2008.