Yulia Navalnaya said on Monday she would continue the fight of her dead husband Alexei Navalny for a "free Russia" and called on supporters to oppose President Vladimir Putin with greater fury than ever.
Navalnaya's call from abroad for resistance to Putin comes less than a month before a presidential election that is almost certain to hand the Kremlin chief another six-year term.
In a nine-minute video message, Navalnaya, 47, accused Putin of killing her husband and said he had cut away half of her heart in doing so and robbed their two children of a father.
"I want to live in a free Russia, I want to build a free Russia," Navalnaya said in the video message entitled "I will continue the work of Alexei Navalny".
"I urge you to stand next to me," she said. "I ask you to share the rage with me. Rage, anger, hatred towards those who dared to kill our future."
It was unclear where she was speaking from but she was not in Russia. Navalnaya attended a meeting of European Union foreign ministers in Brussels on Monday which was weighing imposing further sanctions on Russia over her husband's death.
Britain has also threatened unspecified consequences for Russia. By contrast, Donald Trump, who drew criticism as US president for his praise of Putin, made his first public comment on the death of Navalny but cast no blame.
Navalnaya accused Russian authorities of hiding Navalny's corpse and of waiting for traces of the Novichok nerve agent to disappear from his body. She gave no evidence, but said her team would publish details of who killed her husband.
"Vladimir Putin killed my husband," Navalnaya said. "By killing Alexei, Putin killed half of me - half of my heart and half of my soul.
"But I still have the other half, and it tells me that I have no right to give up. I will continue the work of Alexei Navalny, continue to fight for our country."
Tests on Navalny's body will take 14 days to complete, Ivan Zhdanov, an ally, said on Monday, citing an investigator.
Navalny, 47, fell unconscious and died suddenly on Friday after a walk at the Polar Wolf penal colony above the Arctic Circle where he was serving a three-decade sentence, the prison service said.
The West and Navalny's supporters say Putin is responsible for Navalny's death. The Kremlin has denied involvement and said that Western claims that Putin was responsible were unacceptable.
Putin has made no public comment on Navalny's death but it has further deepened a gaping schism in relations between Moscow and the West caused by the nearly two-year Ukraine war.
Asked by reporters on Monday how Putin had reacted to news of the death, his spokesman Dmitry Peskov said: "I have nothing to add."