By Maria Tsvetkova
MOSCOW (Reuters) - Russian opposition groups said on Tuesday they would hold a rally near the Kremlin next month, stepping up pressure on Prime Minister Vladimir Putin over a controversial election win by his party.
Putin has already faced large opposition protests against his 12-year rule following the December 4 parliamentary vote, which protesters and international observers said was marred by violations and fraud.
Once fractured, opposition groups have been galvanized by discontent over the poll.
Liberal opposition leader Vladimir Ryzhkov said protesters would apply for permission to hold rallies in Moscow and other Russian cities on February 4.
About 17,000 users of Facebook and other social networking services have left messages on the websites saying they plan to attend the Moscow rally.
Despite the protests, Putin is expected to win a presidential election in March and return to the Kremlin.
He has promised that vote will be fair, but has rejected any possibility of annulling the previous parliamentary poll, as demanded by the protesters.
In an attempt to appease protesters' demands, President Dmitry Medvedev, who was steered into office by Putin in 2008, has amended laws to make it easier for political parties to register presidential candidates.
(Reporting By Maria Tsvetkova, additional reporting by Darya Korsunskaya; Writing by Thomas Grove; Editing by Alessandra Rizzo)