Critic of Russia's Putin raises stakes with Moscow race MOSCOW (Reuters) - A prominent critic of Russian President Vladimir Putin, Alexei Navalny, won an opposition party's backing on Friday in a Moscow mayoral election, raising the stakes in a trial that may put him behind bars for a decade. Alexei Navalny, 37, the most prominent leader of a wave of street protests against Putin's 13-year rule that erupted in late 2011, would be barred from the September 8 election if he is convicted in an ongoing trial. "I'm running in the elections to rip our city out of the pincers and tentacles of Mr Putin, Mr Sobyanin and the (pro-Kremlin) United Russia party," Navalny said on Friday before opposition party RPR-PARNAS endorsed his candidacy. Sergei Sobyanin, appointed by the Kremlin in 2010, resigned earlier this month and called for a snap election in what critics said was a Kremlin ploy to legitimize his rule and cheat opponents out of the time needed to mount a challenge. Navalny, who said exiled economist Sergei Guriev will help to draft his platform, vowed to concentrate on improving the healthcare system, regulating migration and improving housing infrastructure. Guriev, a vocal critic of corruption who fled Russia earlier this year after hostile questioning by law enforcement officers, confirmed in an email that he will help Navalny write the economic part of his platform. Anti-graft blogger Navalny faces up to 10 years in jail if found guilty of stealing 16 million roubles ($500,000) from a state timber firm in a trial he says is politically motivated. A June poll by the independent Levada pollster showed only three percent of Muscovites would vote for Navalny versus 45 percent who would cast their ballot for Sobyanin. Navalny could count on more votes if he ends up being the only opposition candidate. Billionaire Mikhail Prokhorov, second to Sobyanin in the poll, said this week that he would not run. Putin critics say the trial of Navalny, who denies any wrongdoing, is part of an intensifying clampdown on dissent. The Western-educated Navalny faces three criminal cases and dozens of other opposition activists also face prosecution. The Kremlin denies cracking down on dissent or exercising political influence over the Russian judiciary to persecute critics of Putin, a 60-year-old former KGB spy. $1 = 32.0225 Russian roubles Reporting by Gennady Novik, Writing by Gabriela Baczynska, Editing by Alexei Anishchuk and Michael Roddy