The UN's human rights expert on Russia has said Moscow is responsible for Alexei Navalny's death.
He was either killed in prison or died from detention conditions that amounted to torture, said Mariana Katzarova.
More people will suffer the same fate as the opposition leader, given the level of repression in Russia, she said.
"Ever since the death of Alexei Navalny, there is no day passing without asking myself, who is the next Navalny?" she said.
"And there will be a next Navalny, for sure, with this level of repression."
Ms Katzarova said Mr Navalny was held in solitary for a total of 300 days, which could have caused "a slow death over several years".
Civil society groups say that around 600-1,000 political prisoners are being detained in Russia for voicing opposition to the Russian invasion of Ukraine or refusing to fight in it.
Ms Katzarova said she was "very worried" about opposition politician Vladimir Kara-Murza, a Russian-British citizen jailed in Siberia on treason charges he rejects.
The Kremlin has rejected accusations Vladimir Putin had Mr Navalny killed, saying he died of natural causes on 16 February in an Arctic penal colony.
