Two human rights advocates were detained on Thursday along with 35 other protesters after staging an anti-government rally in front of Moscow City Hall.
Lev Ponomaryov and Sergei Udaltsov of the For Human Rights and Left Front groups were detained after leading a Day of Wrath protest against Mayor Yuri Luzhkov's policies.
"I was walking through the underpass from the City Hall to the square when I was detained," Udaltsov told RIA Novosti by phone.
A police spokesperson said 35 people were detained.
Protesters were carrying "black marks" - black scraps of paper that symbolize distrust in Luzhkov's policies.
The Moscow authorities said earlier they would thwart any "unauthorized" protests in front of the City Hall.
Around 200 protesters turned out for a March of Dissent rally on nearby Triumfalnaya Square on July 31 to demand freedom of assembly. Some 75 were hauled away in police vehicles.
Nearly 200 protesters were detained during a rally on May 31, causing public outrage, as riot police far outnumbered demonstrators and people with disabilities as well as a World War II veteran were reportedly among those detained.
Opposition groups hold Marches of Dissent on the last day of each 31-day -month in defense of their right to freedom of assembly, as enshrined in Article 31 of the Russian Constitution.
MOSCOWM August 12 (RIA Novosti)
