Russian opposition leader Alexei Navalny was poisoned with a chemical nerve agent Novichok, Germany's government says.
The toxicological reports at Berlin’s Charité hospital showed "unequivocal proof" of an agent from the Novichok group.
Navalny, who fell ill on a flight from the Siberian city of Tomsk to Moscow last month, was transferred to Berlin two days later.
Video footage from the plane showed a man in apparent agony.
The flight made an emergency landing in Omsk, and Navalny was transferred to a hospital there, before his wife and supporters pleaded for him to be taken to Germany for treatment.
German chancellor, Angela Merkel, said in a personal statement on Wednesday afternoon that testing by a special military laboratory had shown proof of a chemical nerve agent from the novichok group.
“It is now clear, Alexei Navalny is the victim of a crime,” Merkel said.
“He was meant to be silenced. This raises very difficult questions that only the Russian government can answer, and has to answer,” she added.
A Novichok nerve agent was used to poison former Russian spy Sergei Skripal and his daughter in the UK in 2018. While they survived, another woman, Dawn Sturgess later died in hospital.
Kremlin spokesman Dmitry Peskov said Wednesday that the Kremlin had not received any information from Germany that Navalny was poisoned with a substance from the Novichok group, according to Russian state news agency TASS.
Russia's Foreign Ministry said it was waiting for a German response to a request from the Russian Prosecutor General regarding Navalny's treatment and diagnosis.
The German government’s official statement described the attack on Navalny with a chemical nerve agent an “astounding act”.
Chancellor Merkel said Germany's Nato and EU partners had been informed of the results of the investigation and they would decide on a common and appropriate response based on Russia's reaction. "The world will be waiting for answers," she added.
Following the German government's announcement on Wednesday, Leonid Volkov, Navalny's chief of staff, tweeted a picture of Russian President Vladimir Putin's signature. In the accompanying text, Volkov wrote that the act of poisoning Navalny with Novichok was akin to leaving an autograph at the scene of the crime.
The Russian doctors who treated Navalny in Siberia have repeatedly contested the German hospital’s conclusion, saying they ruled out poisoning as a diagnosis and that their tests for poisonous substances came back negative.
Navalny is being kept in a medically induced coma and on a ventilator at the intensive care unit of the Charité hospital. While his condition remains serious, a spokesperson said last Friday that he is stable and there was no immediate danger to his life.
What is Novichok?
The name Novichok means "newcomer" in Russian, and applies to a group of advanced nerve agents developed by the Soviet Union in the 1970s and 1980s.
Novichoks were designed to be more toxic than other chemical weapons and some versions begin to take effect rapidly - between 30 seconds to two minutes.
Novichok agents have similar effects to other nerve agents - they act by blocking messages from the nerves to the muscles, causing a collapse of many bodily functions.
The use of novichok nerve agents was banned last year after being added to the chemical weapons convention’s list of controlled substances.