Russian protesters gather outside presidential administration building

Russian riot police ringed off the presidential administration building in Moscow on Saturday after a crowd gathered nearby following an opposition rally and hundreds more were seen walking towards the area chanting, Reuters witnesses said.

Police detained at least 20 people after more than 500 people were seen moving towards the area, many of them young and chanting: "Putin is a thief".

Tens of thousands of Muscovites staged Russia's biggest political protest for years earlier on Saturday, rallying to demand free city-wide elections in spite of a government crackdown.

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