MOSCOW (Reuters) - Thousands of people gathered in Moscow on Saturday to mark five years since the murder of prominent Kremlin critic Boris Nemtsov, joining a march that organizers said was also a protest against planned changes to Russia’s constitution.

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The annual march is the first major demonstration organized by Russia’s political opposition since President Vladimir Putin proposed a raft of changes to the constitution last month.

The proposals, widely seen as a means to allow Putin to extend his grip on power after leaving the presidency, will be put to a national vote on April 22.

Over 22,000 people rallied for Saturday’s march in a square around 2 kilometers (1.24 miles) from the Kremlin, according to White Counter, a monitoring organization that tallies up participants at rallies using metal detector frames.

Demonstrators carried photos of Nemtsov and posters reading “No to eternal Putin” and “No to the usurpation of power”.

Sergei Tsaplienko, an energy worker, said he was joining the march because he wanted freedom and democracy for the Russian people.

“I haven’t got the strength to stand the government’s abuses of power,” he said. “The constitution is about to be transformed into some sort of comic strip.”

Moscow city authorities granted official permission for the march to be held, unlike several unsanctioned protests in the capital last summer that, at their peak, drew around 60,000 people and led to thousands of detentions.

Organizers said that linking the commemorative march with a protest against constitutional reform was consistent with the ideas of Nemtsov, who was one of Putin’s most vocal critics.

“This is exactly what Boris Nemtsov was fighting for, and he gave his life for it,” organizers wrote on the event’s web page.

Nemtsov, a former deputy prime minister, was killed in 2015 as he walked home across a bridge near the Kremlin’s walls. He had been working on a report examining Russia’s role in the conflict in Ukraine.

His allies have criticized the subsequent investigation for failing to identify who ordered the killing, though five men were given jail sentences.

Saturday’s protest featured a banner calling on the authorities to find and prosecute those who orchestrated the killing.

The call was echoed by a representative of the U.S. embassy in Moscow earlier this week, during a visit by foreign diplomats to Nemtsov’s memorial.