Story highlights Putin critic Boris Nemtsov was killed two years ago after promoting an opposition rally

He was an advocate of freedom of speech and democracy

Moscow (CNN) Thousands of Russians packed streets in Moscow on Sunday to mark the second anniversary of Putin critic Boris Nemtsov's death.

Nemtsov, 55, was shot in the back while walking with his Ukrainian girlfriend in central Moscow on February 28, 2015. A gunman in a car opened fire, prompting immediate speculation that the killing was targeted.

A deputy prime minister in the late 1990s under President Boris Yeltsin, Nemtsov had been one of current President Vladimir Putin's most vocal critics.

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Nemtsov was a top official with the Republican Party of Russia/Party of People's Freedom, a liberal opposition group. He was a vocal critic of the Kremlin's handling of the Ukraine crisis.

His life had been threatened on social media weeks before his death, his lawyer said.