Several thousand people have taken to the streets of Moscow shouting “Down with Putin” as international observers in Russia’s parliamentary elections speak of flagrant violations.
Some scuffles were reported and Russian police said they had detained more than 300 protesters.
The OSCE says the vote was slanted in favour of Prime Minister Vladimir Putin’s party, United Russia.
President Dmitry Medvedev insisted the election had been fair and democratic.
However, a White House spokesman, Jay Carney, expressed “serious concerns”.
United Russia won the election, but with a sharp drop in its support, ahead of Mr Putin’s bid to return to the presidency next March.
Electoral officials said the party had just under 50%, down from 64% in 2007.
