"A flash mob is a tactical method, or act, when its participants gather at the same time in a prearranged place, for a single reason to carry out a joint action and then quickly disperse," said Igor Sundiyev, a professor with the Interior Ministry's research institute.
The information is usually distributed by means of e-mail or mobile phone, giving people detailed instructions, where to meet and, in this case, beat someone up which sometimes ends up as a fatal attack.
Sundiyev said that instructions had been traced to sites registered in Canada, Switzerland and Malaysia, although not in Russia.
The expert said that tactics for the flash mobs are widely used by extremist organizations and in Moscow and the surrounding region alone some 300 people had been involved in criminal actions in the last four months.
Sundiyev said that the organizers of such flash mobs are wanted by the Russian Federal Security Service (FSB) that had already arrested 20 people and are trying to trace the other 300.