Alexei Navalny’s widow Yulia has vowed to continue his work to fight for a “free Russia” in a video posted on Monday.
Her voice sometimes shaking with grief and anger, Ms Navalnaya asked viewers to stand alongside her and “share the fury and hate for those who dared to kill our future”.
She also accused the authorities of hiding her husband’s body.
Navalny’s death in prison was announced on 16 February.
The prison authorities at the Siberian penal colony he was being held in said he collapsed following a walk and never regained consciousness.
Navalny, who was the Russian opposition’s most significant leader for the last decade, had been serving a 19-year sentence on charges many viewed as politically motivated.
Now, Ms Navalnaya – who previously mostly shied away from the spotlight – has indicated she might be ready to continue her husband’s political fight for change in Russia.
In her video message, posted on Navalny’s YouTube channel, Ms Navalnaya said: “Three days ago, Vladimir Putin killed my husband Alexei Navalny. Putin killed the father of my children. Putin took away the most important thing I had. The person who was closest to me and whom I loved most.”
She promised to “continue to fight for our country” and added: “We need to use every opportunity – to fight against the war, against corruption, against injustice. To fight for fair elections and freedom of speech. To fight to take our country back. Russia – free, peaceful, happy – the beautiful Russia of the future, of which my husband dreamed so much.”
In the video, Ms Navalnaya also said she knew “exactly why Putin killed Alexei three days ago” and promised to release the information “soon”.
Ms Navalnaya’s address came shortly before she met EU foreign ministers in Brussels.
Navalny’s body has not yet been released to his family, despite his mother and lawyer travelling to the remote penal colony where he was being held as soon as news of his death broke.
Attempts to locate the body have repeatedly been shut down by the prison mortuary and local authorities.
On Monday, the Kremlin said an investigation into Navalny’s death was ongoing and that there were “no results” as of yet.
Later, Navalny’s spokewoman Kira Yarmish said that investigators told Navalny’s mother they would not hand over the body for two weeks while they conduct a “chemical analysis”.
In her video message, Ms Navalnaya said she believed the authorities were waiting for traces of the deadly nerve agent Novichok to disappear from Navalny’s body.
Western leaders have put the blame for Navalny’s death squarely on President Putin.
During a press conference on Monday, EU foreign policy chief Josep Borrell offered his condolences to Ms Navalnaya and said he believed her husband “was slowly murdered in a Russian jail by Putin’s regime”.
US President Joe Biden said Navalny’s death was a “consequence of something that Putin and his thugs did”.
Both the EU and the US have said they are considering new sanctions on Russia following Navalny’s death.
Germany, Sweden and Finland said they were summoning the Russian ambassadors in their capitals.
Kremlin spokesman Dmitry Peskov said comments by Western politicians in regards to Navalny’s death were “arrogant” and “unacceptable”.
Russian prison authorities said at the weekend that Navalny had suffered “sudden death syndrome”.
Hundreds of people in more than 30 cities across Russia were detained at the weekend for attending makeshift memorials to Navalny.
In Moscow, 20 people were sentenced to various amounts of prison time – ranging from one day to nine days – and two people were fined 10,000 rubles (£85).
