Thousands of Russians rallied in central Moscow on Sunday in memory of slain Russian opposition leader Boris Nemtsov, an outspoken critic of President Vladimir Putin who was assassinated in 2015 on a bridge near the Kremlin.

Reuters reports that protesters carried portraits of the slain politician — who criticized Putin's regime as authoritarian and undemocratic — and held banners reading “We have given Russia away to the crooks, it’s time to take it back" and "These bullets are in all of us."

Around 10,600 people gathered for the rally just north of the Kremlin, according to Reuters.

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At the time of his death, Nemtsov was working on a report examining Russia's role in the Ukraine conflict, which has been condemned by NATO and the United Nations.

His killing at the age of 55 sent a chill through opposition circles, reports Reuters.

An officer in the security forces of Chechnya's Kremlin-backed leader Ramzan Kadyrov was convicted of firing the shots that killed Nemtsov and received a 20-year prison term. Four other men convicted of being involved in Nemtsov's slaying received 11 to 19 years.

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At the time, Reuters reports, supporters of the late politician and physicist accused authorities of a cover-up and said the people who ordered his assassination remained at large.

The Associated Press contributed to this report.