The term “flash-mob” was coined in 2003, to describe a group of people who perform a seemingly spontaneous, yet pre-planned public action. Since then, flash-mobs have become an integral part of internet culture, in part because internet is the simplest way to organize such an event.
The first Russian flash-mobs took place ten years ago, directly following ones in the US and Europe, and were organized over LiveJournal  [ru]. Now, of course, it's easier for planners to use social media like Facebook and Twitter. This was the route taken by the organizers of the January 6, 2013 “Snow Battle” flash-mob in St. Petersburg, who set up a VKontakte group  [ru] with the details of an upcoming snowball fight. Over a thousand people planned to attend the event. After assembling, two teams, the “Black Hats” and the “Colorful Hats”, would:
С места предварительной встречи команды идут на поле битвы (его расположение не будет опубликовано в интернете, узнаете лишь на точке сбора), ждут 14 часов 30 минут, встают стенка на стенку […] ну а дальше всё зависит от вас. Или кто-то ест снег из ваших рук, либо вы едите его сами. =)
From the place of assembly the teams walk to the field of battle (its location will not be announced on the internet, only at the assembly spot), wait until 2:30 PM, line up facing each other […] everything else depends on you. Either you make someone else eat snow, or you are the one who eats it. =)
The battlefield ended up being the Field of Mars  [ru], a centrally located and historic square. However, when the participants got there, they ran afoul of a police patrol, who told them to disperse. According to a local news website  [ru], the organizers had tried, but failed, to get a proper permit for the event. Now they were violating the law by holding an “unsanctioned” public assembly, said the police, and furthermore:
Никакой снежной битвы здесь не будет, по той простой причине, как я сказал, что это является правонарушением и мы, как сотрудники полиции, обязаны данное правонарушение пресекать.
There will be no snow battle here, for the simple reason that it is a administrative offence, and we, as police personnel, are required to prevent this offence.
As the crowd sullenly dispersed several people were detained. A video  [ru] of a police officer lecturing a crowd of teenagers through a megaphone quickly made it to YouTube, and the news spread to Twitter and LiveJournal.
LiveJournal user axelus posted a quick write up  [ru] of what happened to ru_marazm  [ru] (ru_stupidity, a LiveJournal community dedicated to preposterous Russian news), where the post quickly sparked debate and so far has almost a thousand comments. Some bloggers have pointed out that the Field of Mars is not a proper place to have a snowball fight anyway, because  [ru]:
на марсовом поле? где захоронения с войн? они что не из Питера эти флэшмобщики? кто же на кладбищах играется? разве только полные идиоты или провокаторы
on the field of mars? where there are war burials? are they not from St. Pete's these flashmobbers? who plays around on cemeteries? maybe complete idiots or provocateurs
Although there are indeed burials and memorials located in the square, calling it a cemetery is an overstatement.
Others joked  [ru] about the reasons for the dispersal:
– а вдруг они слепят снежного Гитлера?
– Хуже. Вдруг они слепят снежного Путина, а потом, кто-нибудь, не дай боже, отломает ему голову.
– what if they make a Hitler snowman?
– Worse. What if they make a Putin snowman, and then someone, god forbid, breaks off his head.
Political public relations guru Gleb Pavlovsky, was exasperated [ru] on Facebook:
Арест за снежки – это даже не Сталин. Со времён придурка императора Павла такого не было! Что за мрачный кретин у нас правит северной столицей?
Arrests for snowballs – that's not even Stalin-level. We haven't seen something like this since the times of the imbecilic Emperor Paul! What kind of gloomy cretin is in charge of the northern capital?
On the other hand, Moscow city council and Coordinating Council of the Opposition member Max Katz was more upbeat  [ru]:
Каждая такая история это шаг к десакрализации власти, к пониманию, что никаких там ужасов нет и что ты можешь и должен идти в политику чтобы всю эту шваль оттуда выкинуть.
Each story like this is another step to desacralizing the government, to an understanding that there is nothing scary there, and that you can and must go into politics to throw all this riffraff out.
There are definitely more stories like this to go around. Just last summer there was a similar incident  [ru] in the same location — that time the flashmobbers who got dispersed and detained were aiming to have a pillow fight. It seems as though some bureaucrats are indeed threatened by spontaneous mobs. In the words of a topical Twitter joke  [ru]:
сегодня ты играешь в снежки – а завтра свергнешь Путина и конституционный строй!
today you play with snowballs – tomorrow you will overthrow Putin and the constitutional order!
There could be a measure of truth behind the sarcasm. After all, if people can self-organize to pillow-fight, what's to stop them from organizing for something more serious?