Twenty-four hours after polls closed in what the ruling party's rivals called post-Soviet Russia's dirtiest vote, some 5,000 people packed a boulevard in central Moscow, denouncing the election as a fraud and chanting, "Putin -- out!" Was it the first sign of a powerful opposition push that could threaten Putin as he prepares to return to the presidency, or an isolated outburst of anger at the entrenched ruling party and its leader? The size of the protest on Monday, unexpected even by its organisers, underscored increasing fatigue with Putin's United Russia party and frustration with the political system he has put in place over 12 years in power. At its core were several longtime opposition leaders with views and agendas that do not always overlap. Among them was Boris Nemtsov, a liberal former cabinet minister who has been shut out of electoral politics during Putin's 2000-2008 presidency and has become one of his fiercest critics. Another, Ilya Yashin, is a scrappy street politician who has faced off against pro-Kremlin government goons. Yevgeniya Chirikova is an ardent environmental activist who galvanised anti-government sentiment last year with a failed fight against a highway cutting through a forest near Moscow. Alexei Navalny, a nationalist, is also a crusading blogger who has challenged some of Russia's biggest companies with corruption allegations and popularised a phrase dismissing United Russia as the "party of swindlers and thieves". For them, the rally was a matter of course, and facing up to a line of police with elbows locked was nothing new.