The man convicted of gunning down Russian opposition leader Boris Nemtsov on a bridge within sight of the Kremlin has been sentenced to 20 years in prison.

Prosecutors had asked for life imprisonment for Zaur Dadayev over the 2015 killing that sent shock waves through Russia's beleaguered opposition.

Prosecutor Maria Semenenko said a decision on whether to appeal against the sentence would be made after studying the judge's decision.

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Ilya Yashin, a close Nemtsov ally, criticised the sentence.

"Of course, it's small – what is 20 years for a human life?" he said, according to news agency Tass.

Four others convicted of involvement in the assassination were sentenced to terms ranging from 11 to 19 years.

Nemtsov was a top opponent of President Vladimir Putin. At the time of his death, he was working on a report about Russia's military involvement in the war in eastern Ukraine and in the annexation of Crimea from Ukraine.

The report was released three months later.

Dadayev was an officer in the security forces of Chechnya's Kremlin-backed leader Ramzan Kadyrov.

Nemtsov's allies have criticised investigators for not considering a possible role of Kadyrov in the killing.

A spokeswoman for Russia's Investigative Committee, the top criminal investigation body, said the collection of evidence is continuing in an attempt to identify the killing's organisers.