In the year of the Arab Spring, governments gyrate between excessive force and wary tolerance when it comes to street protests.
They nervously recall the growing casualty list of regimes that miscalculated their response to demonstrations, most usually by allowing well-publicised brutality on the part of the state security forces.
The rally today in Moscow is a show of strength by the opposition in response to blatant ballot-rigging in the parliamentary elections last Sunday and the police violence that followed. A demonstration of up to 30,000 people is being allowed across the river from the Kremlin. The opposition would like to stage a rally which shows that popular anger against the government has reached an intensity not seen in Russia since the 1990s.
Probably the ingredients that produced the Arab Spring and several "velvet revolutions" over the past decade are not yet present in Russia. The state has made mistakes, but is not so weak as to make it vulnerable to any home-grown Tahrir Square. The opposition has limited experience and is uncertain of its own popular appeal.
But the balance between using force to intimidate, though not so much that police action provokes anger and creates martyrs, is not easy to calculate. In Egypt and Syria, governments thought that by killing a few protesters and beating the rest they could permanently clear the streets; instead they found they had created mass movements.
For all the differences between protests in Moscow and Cairo – and for that matter in New York and Oakland – some developments are the same. The most important of these is that state violence is now invariably recorded by mobile phone cameras and immediately publicised by YouTube in a way that was inconceivable 20 years ago. "They have the tanks, we have the cameras," say Syrian protesters. The balance of power on the street has changed.