Police officers are trying to disperse a protest rally in downtown Moscow, and several prominent opposition leaders have already been detained.
The Triumfalnaya Square rally was not sanctioned by the authorities, and police have also arrested a number of demonstrators, a RIA Novosti correspondent reported from the scene.
The crowd, many of whom are chanting "Freedom! Freedom!" is being pushed back by riot police.
A number of prominent opposition leaders, including Other Russia opposition party leader Eduard Limonov, Left Front opposition movement leader Konstantin Kosyakin and co-chairman of the Solidarity opposition movement Boris Nemtsov, have been taken into custody.
Pyotr Biryukov, an official spokesman for the Moscow police department, said about 400 people gathered for the event, including 300 journalists.
"About 70 opposition activists were detained," he said.
The elderly chair of the Moscow Helsinki human rights group, Lyudmila Alexeyeva, said she left Triumfalnaya Square as the area was too crowded due to the square's reconstruction. The Moscow government closed the area in mid-August, citing plans to build an underground car park there.
"There was a very narrow space left, where it was almost impossible to stand. It was jammed, so I left the square almost immediately. Now I am following the news on the radio," she said.
Russian opposition groups rally on Triumfalnaya Square on the last day of each month that has 31 days in defense of their right to freedom of assembly, as enshrined in Article 31 of the Russian Constitution.
Permission is seldom granted, however, making most opposition rallies "unsanctioned" by the authorities, who frequently send riot police in to deal with protesters. The authorities regularly suggest alternative venues, but these are usually rejected by the opposition.
MOSCOW, August 31 (RIA Novosti)