Kremlin foe Alexei Navalny's parents and other mourners laid flowers at his grave Tuesday to mark 40 days since his death in an Arctic prison, an important Orthodox Christian rite.

The leading domestic opponent of Russian leader Vladimir Putin, died in February in the penal colony where he was held in harsh conditions.

Orthodox Christians believe the soul of the departed leaves Earth on the fortieth day after death.

Navalny's parents Lyudmila and Anatoly, dressed in black, were among those paying homage at Moscow's Borisovo cemetery under the watch of police.

Flowers and wreaths were heaped in front of the wooden cross at his grave.

Navalny's death robbed Russians opposed to Putin of their most charismatic figurehead at a time when authorities have crushed the last remnants of dissent.

The crackdown has intensified since the Kremlin launched its full-scale hostilities against Ukraine in 2022.

"It's terrible to live to see such a man put to death for his beliefs -- he's a hero," pensioner Irina Lukashova, 65, said wiping her tears.

"But I think in the end good will triumph," she told AFP.

Tatyana, a 50-year-old teacher who lost her job over her opposition to the assault on Ukraine, said Navalny had given Russians someone to look up to.

"His behaviour in prison was admired by all of us," she said, not giving her surname.

"When he left us, then first there was a great grief, some hope dies, and on the other hand, he already becomes an untouchable moral reference point."

Artem, 28, who also only gave his first name, said he was still "devastated" by Navalny's death.

"But there is hope for his wife, that Yulia will continue Alexei's work, and for other Russian figures who continue to fight for the beautiful Russia of the future that Alexei wanted to build," he said.

Navalny's widow, Yulia Navalnaya, who has taken over his mantle from exile, posted a song written and performed in his memory by prominent Russian musicians and artists.

The 10-minute music video -- shot in the style of Navalny's favourite cartoon Rick and Morty -- included quotes from him urging Russians not to "give up".

"Today is 40 days since the murder of Alexei," Navalnaya wrote online.

"Thank you very much to everyone who remembers."

Navalnaya has accused Putin of killing her husband, who was earlier poisoned with a nerve agent before being jailed in 2021.

Former KGB officer Putin secured a new six-year term in power in the face of no genuine challenger at elections this month that will make him the longest ruling leader of Russia since Catherine the Great.