Russia\'s opposition on Wednesday promised new rallies contesting the results of elections won by Vladimir Putin\'s party, despite the arrests of hundreds in a police crackdown on a Moscow protest. Riot police in helmets roughly dragged over 550 protestors into detention vans Tuesday evening in central Moscow, in the second protest in as many days organised via the Internet to protest Sunday\'s parliamentary polls. \"Police detained 569 people for an attempt to hold the unsanctioned rally,\" a law enforcement source told the ITAR-TASS news agency. Putin\'s United Russia party won the polls with a sharply reduced majority, amid signs his popularity might be on the wane. The opposition insists that the results would have been even worse for the ruling party if the polls had been run fairly. International organisations have also criticised the conduct of the polls. OSCE-led observers said they were slanted in favour of United Russia and marred by procedural violations. Internet-based protesters vowed further demonstrations despite a warning by police that participants in unsanctioned protests would be arrested. A group \"for honest elections\" said on its Facebook page that a new demonstration would take place in central Moscow on Saturday afternoon. More than 5,000 members of the Facebook group have already promised to attend. Another social networking group, calling itself \"Against the party of swindlers and thieves\" -- the opposition\'s slogan for United Russia -- said protests would now take place every day at 7:00 pm local time. \"When the authorities have stolen honest elections from the people, we can only defend our rights on the street,\" it said. City courts were due to sentence on Wednesday those arrested at the protest the previous day, Moscow Echo radio station reported. A police spokesman said that around 300 were still being held. The authorities sent in hundreds of pro-Kremlin youth activists to fill the Triumfalnaya Ploshad square ahead of the protest. They banged drums and waved flags protected by a police line holding back opposition supporters to the edges of the square.  Kommersant business daily wrote that police detained its reporter at the rally, despite him saying he was a journalist, and kicked and stamped on him before releasing him. Police in Putin\'s native city of Saint Petersburg reported making around 200 arrests at protests there. The authorities were caught by surprise when the first opposition rally on Monday evening attracted thousands to march through central Moscow, with police detaining more than 300. Influential opposition blogger Alexei Navalny, who coined the slogan \"swindlers and thieves\" to describe United Russia, was among those arrested and sentenced Tuesday to 15 days in a police cell for disobeying police orders. Russian media commented on a news blackout on the protests on state television. Kommersant FM radio station played the headlines from news shows on Channel One and Rossiya 24 channels, making no mention of the rallies. Commentators said the situation was of the authorities\' own making and now their only option to prevent further unrest was to enter into a real dialogue with the protesters. \"We prevented the real political process and built cardboard scenery instead. But it\'s dangerous to hold back a natural process: now it\'s payback time,\" wrote liberal daily Vedomosti in an editorial. City police said that more than 51,000 police were guarding the city streets, among them 2,000 army conscripts, in a heightened security regime launched ahead of the elections. The ruling party polled just under 50 percent of the vote after winning more than 64 percent in 2007 but still held on to an absolute majority of seats in parliament. EU foreign policy chief Catherine Ashton said Tuesday the elections were a cause for \"serious concern\" while US Secretary of State Hillary Clinton said the polls were neither free nor fair.