Russia is bracing itself for the first nationwide protest against Vladimir Putin's 12-year rule amid signs of swelling anger over a poll won by his ruling United Russia party with the alleged help of widescale fraud.

Moscow authorities gave permission for 30,000 people to gather on a square across the river from the Kremlin at 10:00 GMT after detaining some 1,600 activists over the past few days who joined unsanctioned rallies against the December 4 vote.

The opposition is also organising rallies in at least 14 other major cities in a rare outpouring of mistrust in a system put in place by Putin when he first became president in 2000.

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