Condemnation swept in after the death of jailed Russian opposition leader Alexei Navalny was announced on Friday by Russia's prison service. He was 47.
Mr Navalny fell ill after a walk at the Arctic prison colony where he was serving a 19-year term, the federal penitentiary service said.
US President Joe Biden said he was "not surprised, but outraged" and blamed Russia's President Vladimir Putin for Mr Navalny's death.
"Russian authorities are going to tell their own story," Mr Biden said from the White House.
"Make no mistake ... Putin is responsible for Navalny's death."
A moment's silence for the Russian opposition leader was held at the Munich Security Council gathering of 800 leading security and political figures.
Christoph Heusgen, the event's director, revealed that Mr Navalny's widow, Yulia, was at the conference.
“He died just a few minutes ago,” he told the opening ceremony.
“A very special man, our thoughts are today with his wife and his children. Yulia Navalnaya is today in Munich where she wanted to attend this conference and talk about a better Russia.”
Navalny's spokeswoman Kira Yarmysh said there is "almost no hope" that Russian opposition leader Alexei Navalny is alive.
A spokesman for the Kremlin said Mr Putin had been informed of the death of Mr Navalny, who was the foremost political rival of the Russian leader and his regime.
Ms Navalnaya gave an emotional address in which she said she weighed the instinct to fly home to be with her children against the offer to speak to the meeting. In the end, while noting the news came only from the Russian government, she thought, what would her husband have done?
"I want that Putin, and his entire circle, Putin’s friends, his administration, to know, that they will carry responsibility for what they have done with our country, with my family, and with my husband," she said.
The widow spoke after US Vice President Kamala Harris said Navalny's death was “a further sign of Putin's brutality” and held Russia responsible in a keynote address at the conference.
"If confirmed, this would be a further sign of Putin's brutality," she said. "Whatever story they tell, let us be clear, Russia is responsible."
Ms Harris was leading a US delegation that included Secretary of State Antony Blinken, who said he would raise the issue with his counterparts while in Munich.
Ms Harris and Ms Navalnaya met on the margins of the conference, where the Vice President expressed her "sorrow and outrage" in person, a White House official said.
Nato Secretary General Jens Stoltenberg expressed his shock at the news.
“We need to establish all the facts, and Russia needs to answer all the serious questions about the circumstances of his death,” Mr Stoltenberg said.
Ursula von der Leyen, the head of the European Commission, said the news was an abrupt reminder of the need for unity among democrats.
“Deeply disturbed and saddened by news of the death of Alexei Navalny,” she said. “Putin fears nothing more than dissent from his own people. A grim reminder of what Putin and his regime are all about.”
European Council President Charles Michel said Mr Navalny “fought for the values of freedom and democracy”.
“For his ideals, he made the ultimate sacrifice,” he wrote on X.
British Prime Minister Rishi Sunak said the death of the “fiercest advocate for Russian democracy” is “terrible news” and a “huge tragedy” for the people of Russia.
German Chancellor Olaf Scholz said he was “very sad” about the reports.
Latvian President Edgars Rinkevics went further, claiming that Mr Navalny had been “brutally murdered''.
Officials said he died after exercise at the facility where he was being held.
“Resuscitation measures were carried out, which did not yield positive results,” the prison service said. “Paramedics confirmed the death of the convict. The causes of death are being established.”
Russia's Investigative Committee said it has launched a procedural investigation into the death.
Mr Navalny was moved in December from a prison in the Vladimir region of central Russia to a “special regime” penal colony – the highest security level of prisons in Russia – above the Arctic Circle.
His allies decried the transfer to a colony in the town of Kharp, in the Yamalo-Nenets region about 1,900km north-east of Moscow, as yet another attempt to force him into silence.
The town is about 100km from Vorkuta, whose coal mines were part of the Soviet gulag prison-camp system. The remote region is notorious for its long and severe winters.
Mr Navalny had been behind bars since January 2021, when he returned to Moscow after recuperating in Germany from nerve agent poisoning that he blamed on the Kremlin.
Before his arrest, he campaigned against official corruption, organised major anti-Kremlin protests and ran for public office.
He had since received three prison sentences, all of which he rejected as politically motivated. His latest sentence was on charges of extremism.
His lawyers expressed concern in early December after claiming they had not been able to reach him, saying he had been ill and was reportedly being denied food and kept in an unventilated cell.
He was found three weeks later at the prison colony in Kharp.
His allies had accused the authorities of slowly poisoning him, perhaps through prison food, causing him to rapidly lose weight.
Navalny, a 2022 film about the activist’s poisoning and imprisonment, was awarded the Academy Award for best documentary in March last year.
His wife and their two children, Darya and Zakhar, attended the ceremony in Los Angeles and joined the film's director on stage to accept the award.
“My husband is in prison just for telling the truth. My husband is in prison just for defending democracy,” Ms Navalnaya said.
“Alexei, I am dreaming of the day when you will be free, and our country will be free. Stay strong, my love.”
The son of a Soviet army officer, Mr Navalny was born near Moscow on June 4, 1976, and grew up in a series of closed military towns.
Active in municipal politics early in his career, he participated in nationalist rallies and recorded anti-immigration videos that resurfaced as part of attempts to discredit him after his 2021 imprisonment.
