More than 500 hundred people, including opposition leaders and city council election candidates, were arrested on Saturday as police cracked down on an unsanctioned demonstration for fair elections.Hundreds of riot police and members of the national guard were dispatched to the centre of the capital, closing down streets near the planned protest site at city hall and re-routing demonstrators and bystanders.The number of detentions, monitored by the independent OVD-Info group, was still growing after about 3500 people gathered. Prominent political figures were forcefully put into vans before the protest was able to start amid scenes of scuffles.The protest was the second in one week in Moscow after last Saturday's authorised rally for free local elections that around 20 000 people attended. It was one of the largest demonstrations in Russia since the 2011-12 rallies against President Vladimir Putin's re-election. Opposition figure Alexey Navalny, who was at the forefront of last week's demonstration, called for the second round of action in front of city hall on Saturday. The anti-corruption lawyer has not been able to attend, though, as he was arrested on Wednesday and given 30 days in jail.Other young liberal leaders and candidates for the City Duma, Moscow's council elections, such as Dmitry Gudkov and Lyubov Sobol were put into custody after they arrived on site.Country-wide discontentThe recent wave of Moscow protests, which began with a round of smaller rallies between July 15-19, was sparked by the decision of the Russian election committee - strongly linked to central power - to ban all but a few opposition and independent candidates vying for the elections.The move led to a campaign on social media among Moscow residents and activists to take to the streets.The pushback also comes against a backdrop of growing discontent nationwide as Putin's approval ratings slip.The northern region of Arkhangelsk has been the theatre of a year-long battle and occupation by locals of the site of a huge new landfill to be built in the middle of an untouched forest. In the Urals metropolis of Yekaterinburg, thousands demonstrated for several days in May against the building of a new cathedral at a local park.Given this climate of citizen anger and grassroots movements popping up in various regions, authorities have become wary of political opposition gaining any more traction as shown by Saturday's crackdown.