Egyptian military police arrested three activists who were putting up posters in central Cairo on Thursday, a day ahead of a mass rally calling for faster reforms, DPA reported.
The posters promoting Friday's protest were critical of the Supreme Council of the Armed Forces (SCAF), which has been in control of the country since the ouster of president Hosny Mubarak in February.
Lawyers said they were not permitted to see the activists, who have not been formally charged, at the military investigations office where they were taken.
Artist and graphic designer Mohamed Fahmy, who designed the posters and has organized a number of graffiti campaigns around the city, spread news of his arrest along with two others on the internet before his mobile phone was switched off.
Another activist with the April 6 Youth Movement was arrested Wednesday night for putting up posters promoting the protest scheduled to take place in Tahrir Square, the focal point of the uprising earlier this year.
"Detaining activists while leafleting for protests is similar to what used to take place during Mubarak's era," the April 6 movement said in a statement.
Friday's protest, advertised widely on Facebook and Twitter, calls for an end to trials of civilians in military courts, and faster movement on economic and political reforms.
Hundreds of demonstrators have been arrested since Egypt's military rulers and caretaker government banned all protests and strikes in March.
Rights groups have condemned the frequent trials of protesters and other civilians in military courts in Egypt in recent months, saying detainees are often abused in custody, denied access to legal representation and trial proceedings, and receive excessively harsh sentences.