MOSCOW - 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 but jailed his brother in a controversial fraud case.

In a lightning hearing that was abruptly brought forward by two weeks, a judge found both Navalny and his brother Oleg guilty of embezzlement and sentenced the siblings 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’s critic ahead of 2018 presidential elections 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,” Navalny said angrily outside the courthouse, 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 had already been planning to gather later on Tuesday near the Kremlin and by midday, 17,000 people have pledged on Facebook to attend the 1600 GMT rally. The protest has not received required authorisation from city hall and police warned that all illegal activity would be punished.

The charismatic Navalny has become a major thorn in the Kremlin’s side over the last several 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 mayor’s 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 a rushed affair - first the court abruptly moved it forward two weeks to just before the New Year - Russia’s biggest holiday - in a move seen as a tactic to avoid massive protests.

And the reading itself took only about 15 minutes - unusually for Russia where judges usually read sentences for hours, outlining the prosecution’s proof and witness testimonies.

“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 approval from Putin personally. His spokesman Dmitry Peskov told Kommersant FM radio on Tuesday that the president merely follows the case on the media.

In a country dominated by Putin for years, observers say that Navalny is the one figure that can pose a threat to the strongman - a talented orator, he is young, handsome, with a photogenic wife and two kids, an unassuming middle-class lifestyle and is unstained by any ties to the 1990s political scene that many Russians despise.

The maverick politician - who has said he intends to run in the 2018 presidential elections - has seen a half a dozen criminal cases lodged against him and his allies recently, which he says are politically motivated, and has been under house arrest for nearly a year because of separate suspended sentence in a different lawsuit.

Navalny’s allies saw Tuesday’s sentence as an attempt to muzzle the charismatic figure as the ruble plunges and the economy teeters on the edge of recession amid Moscow’s standoff with the West over the Ukraine crisis.

“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 (presidential) 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 in an emailed statement. “The Kremlin seems to be telling independent voices to expect a harsher crackdown in 2015,” the rights organisation said.

Navalny’s sentencing hearing was originally due to take place on January 15 and was abruptly moved forward on Monday after some 15,000 people pledged to attend a rally on that day.