Moscow, Russia - Late Russian opposition leader Alexei Navalny was laid to rest on Friday in a Moscow cemetery where thousands of mourners had gathered, two weeks after he died in an Arctic prison.

The anti-corruption campaigner, President Vladimir Putin's most prominent opponent, was buried after a brief candle-lit funeral service in a nearby church.

The casket was left open in accordance with Russian Orthodox tradition but was quickly closed after the religious service where Navalny's parents could be seen.

At the cemetery, Navalny's coffin was lowered into the grave to the soundtrack of the film Terminator 2 which his spokeswoman said was the 47-year-old's favorite movie.

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Navalny's death has been condemned by Western leaders, and his supporters have accused Putin of murder and of trying to prevent a dignified public burial.

The Kremlin, which has denied involvement and dismissed the accusations as "hysterical," warned against "unauthorized" protests around the funeral.

"We won't forget you!" and "Forgive us!" some mourners shouted, applauding as the coffin arrived for burial.

Thousands then filed past the grave to pay their last respects.

Nearby, a few hundred people could be heard shouting anti-war slogans.

Navalny's widow, Yulia Navalnaya also posted an emotional tribute on social media.

"I don't know how to live without you, but I will try my best to make you up there happy for me and proud of me," she wrote, thanking him for "love, for always supporting me, for making me laugh even from prison, for always thinking about me."

Some 400 mourners have been detained at Navalny memorials since his death, rights organization OVD-Info has said, and more detentions were feared at the funeral, where a heavy police presence could be seen.

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"What are they afraid of?" one mourner, Anna Stepanova, told AFP outside the church. "They are so afraid themselves. The people who came here, they are not scared. Alexei wasn't either."

The French, German, and US ambassadors were seen among mourners outside the church, as were some of Russia's last free independent politicians.

As far as Putin is concerned, Kremlin spokesperson Dmitry Peskov said he had "nothing to say" to Navalny's family.