Russia's top opposition leader Alexei Navalny on Tuesday called for mass protests to "destroy" President Vladimir Putin's regime after a court handed him a suspended sentence and jailed his brother in a controversial fraud case. At a lightning hearing that was abruptly brought forward by two weeks, a Moscow judge found both Navalny and his brother Oleg guilty of embezzlement and sentenced them to three and a half years in what is widely seen as a politically motivated case. But while Navalny's sentence was suspended, his younger brother, who is not involved in politics, was ordered to serve the time behind bars in what observers saw as an attempt to muzzle the Kremlin critic ahead of the 2018 presidential election by taking his brother hostage. "This regime does not just destroy its political opponents... now they target, torture and torment the relatives of its political opponents," the 38-year-old Navalny said outside court, calling the verdict "the most mean and disgusting" possible. "This regime has no right to exist, it must be destroyed," he said. "I call on everyone to take to the streets today." Navalny's supporters plan to gather near the Kremlin from 1600 GMT and by midday, 18,000 people had pledged on Facebook to attend. The protest has not received the required authorisation from city hall, which ominously warned that "all unsanctioned actions will be prevented by the security forces". Trucks carrying police were seen in central Moscow and news agencies reported that exits from the metro to Manezhnaya square -- the location of the planned rally -- will be closed after 1500 GMT. - Thorn in Kremlin's side - The charismatic Navalny has become a major thorn in the Kremlin's side over the last few years, first building a massive support base on the Internet as an anti-corruption blogger, then rallying tens of thousands during the 2011-12 anti-Putin protests and most recently coming in second in last year's Moscow mayoral race after a grassroots campaign against the Kremlin-backed candidate. The Navalny brothers were accused of defrauding French cosmetics company Yves Rocher of nearly 27 million rubles (more than half a million dollars at the exchange rate at the time), although the firm has said that it suffered no damages. Prosecutors had asked the court to jail Alexei for 10 years and Oleg for eight. Tuesday's hearing was brought forward two weeks in a move seen as a tactic to avoid massive protests and make it impossible to request authorisation for rallies. The session took only about 15 minutes -- unusual for Russia where judges usually read sentences for hours. "What are you jailing him for, what sort of disgrace is this? This is to punish me even more?" Navalny yelled, slamming his fists on the table, as the judge announced that his 31-year-old brother, a father of two young children, would be jailed. Observers say that because of Navalny's prominence the verdict could not have been issued without Putin's personal approval. However, his spokesman Dmitry Peskov told Kommersant FM radio that Putin merely followed the case in the media. Germany's human rights commissioner Christoph Straesser said the ruling was "another blow against outspoken civil society" in Russia and called for Moscow to investigate the criticism of the case and to allow peaceful protests. - Brother taken 'hostage' - In a country dominated by Putin for years, observers see Navalny as the one figure who can pose a threat to the strongman. A talented orator, he is young, handsome, with a photogenic wife and two children, an unassuming middle-class lifestyle and he lacks ties to the 1990s political scene that many Russians despise. The maverick politician -- who has said he intends to run in the 2018 election -- has seen a half a dozen criminal cases lodged against him and his allies, which he says are politically motivated. He has been under house arrest for nearly a year because of separate suspended sentence in another case. Navalny's allies saw Tuesday's sentence as an attempt to silence him as the ruble plunges and the economy teeters on the edge of recession against the backdrop of Moscow's standoff with the West over Ukraine. "In essence, Oleg has been taken hostage, and Navalny will get discredited due to innocent people sent to jail because of him," opposition politician Boris Nemtsov wrote on Facebook. "They hope to control Alexei Navalny's political activity... in the years before the 2018 elections" while still keeping the possibility of jailing Navalny himself later, his ally Leonid Volkov wrote. The decision is also aimed at "intimidating other critics of the government," said Human Rights Watch. "The Kremlin seems to be telling independent voices to expect a harsher crackdown in 2015."