A day after he was sentenced to 20 days in jail, the Kremlin warned on Tuesday supporters of Russian opposition leader Alexei Navalny against holding unsanctioned rallies. The organizers of such events would be prosecuted, it warned. Navalny was sentenced on Monday to a third term in jail for calling for an unsanctioned protest. The ruling would keep him away from a major rally this weekend. Police detained Navalny on Friday, preventing him from traveling to a rally in a major Russian city that had given its official permission to hold the gathering. Charges brought against the Kremlins top rival relate to the upcoming rally in St. Petersburg, Russias second-largest city and President Vladimir Putins hometown, which has not been sanctioned. After he announced his presidential bid last year, Navalny, arguably Russias most popular opposition politician, inspired a grassroots campaign in Russian regions to support his nomination. "20 days in jail. Old man Putin got so scared of our rallies in the regions and decided to make himself a little present for himself for his birthday," Navalny tweeted shortly after the ruling Monday evening. The rally in St. Petersburg was scheduled for Saturday, which is also Putins birthday. Navalnys campaign late Monday called for rallies to protest his arrest in other Russian cities this Saturday. The Kremlin warned Navalnys supporters to refrain from holding any unsanctioned rallies in his absence. "Public calls for unsanctioned events and rallies run counter to the legislation in force and, certainly, their organizers will be prosecuted," Kremlin spokesman Dmitry Peskov told a conference call with reporters. A Russian law on public gatherings, which was hastily adopted following massive anti-government rallies in 2011-2012, carries 30 days in jail for repeated violations. In another Moscow courthouse, a judge is expected to hand down a ruling later Monday in the case of Navalnys campaign chief, Leonid Volkov, who faces similar charges. The Kremlin has dismissed Navalny, who has faced repeated jailings and criminal cases, as an urbanite out of touch with people living in Russias 11 time zones where Putin draws his support from. Yet that began to change earlier this year when Navalny, a 41-year-old lawyer, opened campaign offices in 80 cities and towns. Most of those places had not seen a diverse political life for decades, and Navalny attracted thousands of supporters.