The United States and several European governments have demanded the release of opposition politician Alexei Navalny from Russian detention.
Navalny, 44, was detained by police soon after his flight from Germany landed in Moscow on Sunday.
The activist was returning to the country five months after he was almost killed in a nerve-agent attack he blamed on Russian authorities.
President Vladimir Putin's government denies poisoning Navalny.
The opposition politician's allegations have however been backed up by reports from investigative journalists.
Reacting to his arrest, the US and EU led calls for Navalny - an arch critic of President Putin - to be freed, but stopped short of threatening any punitive action.
The US Secretary of State, Mike Pompeo, said the Russian authorities were trying to silence their critics. He called for Navalny's "immediate and unconditional release".
"Confident political leaders do not fear competing voices, nor commit violence against or wrongfully detain political opponents," Pompeo said.
US President-elect Joe Biden's incoming national security adviser stuck a similar tone. "The Kremlin's attacks on Mr Navalny are not just a violation of human rights, but an affront to the Russian people who want their voices heard," Jake Sullivan said.
The response from the European Union was no less scathing, with France, Italy and the president of the European Council, Charles Michel, issuing demands for Navalny's release.
In its statement, the UK government said it was "deeply concerned" by Navalny's arrest, adding: "Instead of persecuting the victim of this terrible crime, the Russian authorities should investigate how a chemical weapon came to be used on Russian soil."
What happened in the lead-up to the arrest?
When Navalny was poisoned last August and collapsed on an internal flight in Siberia, he was flown to Germany for emergency medical treatment. As he recovered, he said he intended to return to Russia.
On Sunday he made good on that pledge, boarding a Pobeda Airlines flight in Berlin despite warnings he would face arrest on landing.
The plane was packed with journalists, including Andrey Kozenko of the BBC Russian Service. Shortly before landing, the pilot announced that for "technical reasons", the plane was being diverted from Vnukovo airport, where thousands of Navalny supporters had gathered, to Sheremetyevo airport, causing a stir among the passengers.
"I know that I'm right. I fear nothing," Navalny said upon landing, just minutes before he was detained. "Have you been waiting for me long?" he asked border guards.
He kissed his wife Yulia - who had flown with him from Germany - after police officers warned they would use physical force if he disobeyed their orders to come with them. Despite pleas, Navalny's lawyer was not allowed to accompany him.
The activist was later taken to a police station in Moscow, where he spent the night.
What was the reason given for his detention?
In a statement late on Sunday, Russia's prison service said the opposition leader "had been wanted since 29 December 2020 for repeated violations of the probation period". It added that he would remain in custody until a court decision.
The authorities accuse him of violating conditions imposed after a conviction for embezzlement, for which he received a suspended sentence. He has always said the case was politically motivated.
Separately, Russian prosecutors have launched a new criminal case against Navalny on fraud charges related to transfers of money to various charities, including his Anti-Corruption Foundation.
The activist accuses Putin of targeting him with spurious cases.