There is "unequivocal proof" that Russian opposition politician Alexei Navalny was poisoned with a Novichok nerve agent, Germany has said.
Chancellor Angela Merkel said he was a victim of attempted murder and the world would look to Russia for answers.
Mr Navalny was flown to Berlin after falling ill on a flight in Siberia last month, and remains in a coma.
His team says he was poisoned on President Vladimir Putin's orders. The Kremlin has dismissed the allegation.
The Kremlin spokesman called on Germany for a full exchange of information and foreign ministry spokeswoman Maria Zakharova complained the Novichok allegations were not backed up by evidence. "Where are the facts, where are the formulas, at least some kind of information?" she asked.
A Novichok nerve agent was used to poison former Russian spy Sergei Skripal and his daughter in the UK in 2018. While they survived, a British woman later died in hospital. The UK accused Russia's military intelligence of carrying out that attack.
Prime Minister Boris Johnson condemned the latest attack as "outrageous". "The Russian government must now explain what happened to Mr Navalny - we will work with international partners to ensure justice is done," he tweeted.
What has Germany said?
After the government in Berlin released results of toxicology tests carried out at a military laboratory, Chancellor Merkel said there were now "serious questions that only the Russian government can and must answer".
"Someone tried to silence [Mr Navalny] and in the name of the whole German government I condemn that in the strongest terms."
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.
Mr Navalny's wife Yulia Navalnaya and Russia's ambassador to Germany would also be informed of the findings, the Berlin government said.
The European Union has demanded a "transparent" investigation by the Russian government. "Those responsible must be brought to justice," a statement read. Nato Secretary-General Jens Stoltenberg also called for an inquiry in a tweet, while the US National Security Council (NSC) said the suspected poisoning was "completely reprehensible".
"We will work with allies and the international community to hold those in Russia accountable, wherever the evidence leads, and restrict funds for their malign activities," an NSC spokesman said.