Russia’s central election commission on Monday rejected the registration of the opposition leader’s candidacy for the country’s upcoming presidential race.

According to local media, 12 out of 15 commission members voted against Alexei Navalny’s bid to register as a candidate in the March 2018 election to run against popular incumbent Vladimir Putin.

Navalny, who leads the anti-corruption Progress Party, said he would appeal the decision before Russia’s constitutional court, adding that he was nonetheless “aware that the court is part of the same system.”

A prominent Putin critic, Navalny has been arrested multiple times by Russian authorities, who have charged him with several counts of embezzlement and fraud; this led to a conviction in 2013 and another in 2014, though the prison sentences were later suspended.

Due to his official criminal record, the election commission decided to deny Navalny his application by arguing that he had lost his right to passive suffrage following the convictions.

Thousands of Navalny’s followers on Sunday had taken to the streets in several cities throughout Russia to support his candidacy.