Tens of thousands of Russians are expected to gather in Moscow Sunday to honor the life of Boris Nemtsov, a Russian opposition leader who was shot dead Friday. The event will replace an earlier planned political march Nemtsov had rallied support for just hours before he was killed.  

Sunday’s event is scheduled to begin at 3 p.m. local time (7 a.m. EST), and Russian authorities approved a permit for up to 50,000 people to participate, according to Voice of America.

Nemtsov, 55, was walking across a bridge over the Moscow River, near the Kremlin, when unidentified gunmen in a car shot him four times in the back, Russia’s interior ministry said Friday. At the time, Nemtsov was accompanied by a Ukrainian acquaintance who was unharmed in the incident.

On Saturday, hundreds of Russians flocked to the bridge where Nemtsov was killed to pay respects and lay flowers in remembrance of the political leader.

His death has rattled Russia’s political opposition. Nemtsov, formerly Russia’s deputy prime minister, was one of the country's most prominent opposition figures. He had openly criticized Russian President Vladimir Putin and Russia’s involvement in the conflict in eastern Ukraine. Ukrainian President Petro Poroshenko said publicly that Nemtsov had been working on a report detailing evidence of Russia’s direct involvement in the conflict, something that Moscow had always denied even though it acknowledged support for Ukraine’s separatists. Sunday’s rally was originally aimed at protesting both the Ukraine conflict and Russia’s mounting economic problems.

Poroshenko told news service Interfax that the report Nemtsov was working on may have factored into the killing. “Boris declared that he must show convincing proof of Russian troops’ participation in Ukraine,” Poroshenko said, according to Bloomberg. “Someone was afraid of that very much. Boris wasn’t afraid, but his executioners were.”

The perpetrator and motive behind Netmov’s killing are still unknown. The BBC reported that Putin sent a letter offering condolences to Nemtsov’s mother and pledged that the government would “do everything to ensure that the perpetrators of this vile and cynical crime and those who stand behind them are properly punished.”

A government spokesman, Vladimir Markin, said the Kremlin would take up the investigation into Nemtsov’s murder, the Associated Press reported. Markin said Saturday that an investigative committee was looking into the possibility that the killing may have been a “provocation to destabilize the political situation in Russia,” using Nemtsov as a “sacrificial victim” to that aim.