Several thousand supporters of Azerbaijani opposition presidential candidate Rustam Ibragimbekov rallied outside the capital, Baku, on August 18.
At the sanctioned event, which took place some 10 kilometers from the city center, supporters chanted slogans such as "Do not steal our votes!" and "Do not test our patience!"
According to organizers and activists, police detained at least 11 activists ahead of the rally.
Activists also allege that public transport to the rally site was restricted by authorities.
A presidential election is scheduled for October 9.
Ibragimbekov, a 74-year-old Oscar-winning screenwriter, was chosen last month as a presidential candidate by a coalition of Azerbaijan's main opposition groups -- the National Council of Democratic Forces -- to challenge incumbent President Ilham Aliyev.
But Ibragimbekov is still awaiting confirmation from Moscow that his Russian citizenship has been stripped in order to be allowed to run in the election.