WARSAW, Poland—Alexei Navalny's widow Yulia Navalnaya on Monday accused President Vladimir Putin of killing her husband and vowed to continue his work, three days after he died in a Russian Arctic prison.

Her video address came as the Kremlin said an investigation into his death was "ongoing" and his mother Lyudmila was denied access to his body for a third day.

"Three days ago, Vladimir Putin killed my husband Alexei Navalny," Navalnaya said in a video address on Navalny's YouTube channel.

"Alexei died in a prison colony after three years of torment and torture," she said.

Her address came shortly before she was due to meet EU foreign ministers.

"I will continue the work of Alexei Navalny. I will continue to fight for the freedom of our country,"  Navalnaya said. "And I call on you to stand by me."

"Putin took from me the most valuable thing that I had, the closest and most loved person. But Putin also took Navalny from you," the 47-year-old said.

Navalnaya stood by her husband as he galvanised mass protests in Russia, flying him out of the country when he was poisoned before defiantly returning to Moscow with him in 2021, knowing he would be jailed.

The announcement she will replace Navalny is a momentous and unpredictable turn for Russia's exiled and beleaguered opposition, which has been left leaderless after Navalny's death.

It comes just one month ahead of a presidential election in Russia, with Putin running with no real challengers.

Navalnaya said her husband's team knew "concretely for what" Navalny was killed and said they would work to "find out the names" of those who they say murdered him.

The Kremlin earlier on Monday said there were "no results" from an "ongoing" Russian investigation and slammed Western accusations.

"At the moment, the results of the investigation have not been released, they are unknown," Kremlin spokesman Dmitry Peskov said.

He decried Western statements that the Kremlin was responsible for Navalny's end as "absolutely unacceptable."

Peskov refused to say how Putin -- who has not commented on the death despite making public appearances since -- reacted to his opponent dying.

He also refused to say when the body will be handed over.

- Mother denied access to morgue -
Navalny's allies said his mother Lyudmila was again denied access to a morgue in the Russian Far North.

"Alexei's mother and his lawyers arrived at the morgue early in the morning. They were not allowed to go in," Navalny's spokeswoman Kira Yarmysh said on social media.

"One of the lawyers was literally pushed out. When the staff was asked if Alexei's body was there, they did not answer," she added.

The mother has not spoken publicly and Russian officials have given no timeframe of when the body will be handed over.

Navalny's team have accused authorities of dragging out the process to "cover up their tracks".

Russian authorities detained hundreds of mourners who brought flowers to local monuments across the country over the weekend, rights groups said.

In Moscow on Monday, AFP saw several people bring flowers to a monument to Soviet repression in memory of Navalny. One woman stood and cried, with a heavy police presence nearby.

"One for all," read one note, quoting a slogan Navalny often used at protests.

Outside Russia, Russian emigres held vigils in European cities, where hundreds of thousands fled to after Moscow launched its Ukraine offensive.

- Rock star tribute -

In Kazakhstan -- another country where many Russians fled to -- Russian rock legend Yuri Shevchuk performed a song in honour of Navalny on Sunday.

"Alexei Navalny who spoke to us, Russians, about freedom and who reminded us all that we could be free in the best sense of the word," Shevchuk told a crowd to applause, according to Russian independent media.

Shevchuk played at mass anti-Putin rallies in Moscow in 2011, which thrust Navalny to the forefront of the Russian opposition movement.

Russia's prison service said in a short statement on Friday that Navalny had died "after a walk" in the IK-3 colony in the Arctic Yamal region -- known as the "Polar Wolf."

Navalny had continued from behind bars to call on Russians to fight against the regime, calling on them "not to be afraid."

He also watched on in despair as Russia launched its Ukraine offensive, denouncing it in one statement as "stupid and senseless".