MOSCOW (Reuters) - A Russian court sentenced opposition leader Alexei Navalny to his third jail term this year on Monday, disrupting plans by the long-time Kremlin critic to campaign before Russia s coming election. Russia holds a presidential election in March next year. The incumbent Vladimir Putin has yet to announce his candidacy but is widely expected to do so and to win. Navalny hopes to run, even though Russia s central election commission declared him ineligible because of a suspended prison sentence, which he says was politically motivated. He was detained by police on Friday as he left his Moscow home to attend a pre-election rally in a provincial town and later charged with repeatedly violating laws governing the organisation of public meetings and rallies. A court sentenced him to 20 days in jail on Monday, a term which means he will miss a campaign rally planned in Putin s hometown of St Petersburg on Oct. 7 - the Russian leader s birthday.   Old man Putin is so scared of our meetings in the regions that he decided to make himself happy with a small gift for his jubilee,  Navalny said on Twitter after his sentencing. Navalny has been jailed twice already this year after organising rallies and protests against government corruption. Russian opposition activists say they are coming under increased pressure as the election approaches, with authorities  frequently turning a blind eye to violent attacks. (This version of the story refiles to fix typo in paragraph three) 