The United States and several European governments have demanded the release of opposition politician Alexei Navalny from Russian detention.
Mr Navalny, 44, was detained by police soon after his flight from Germany landed in Moscow on Sunday.
He was returning to the country five months after he was almost killed in a nerve-agent attack he blamed on Russian authorities. Moscow denied any role.
Russia’s foreign minister has dismissed the international condemnation.
Sergei Lavrov said Western politicians were using it as a way to “divert attention” from domestic problems.
Russia’s prison service on Sunday said the Kremlin critic had violated the terms of his suspended sentence for embezzlement. Officials said he would remain in custody until a court ruling.
Lawyers for Mr Navalny say they have not been granted access to him.
What has the international reaction been?
The US and European Union have led calls for Mr Navalny to be freed, but have stopped short of threatening any punitive action.
US Secretary of State Mike Pompeo said the Russian authorities were trying to silence their critics. He called for Mr Navalny’s “immediate and unconditional release”.
“Confident political leaders do not fear competing voices, nor commit violence against or wrongfully detain political opponents,” Mr 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 EU was equally strong, with France, Italy and the president of the European Commission, Ursula von der Leyen, among those issuing demands for his release.
German Foreign Minister Heiko Maas described the arrest as “totally incomprehensible”.
UK Foreign Secretary Dominic Raab said Mr Navalny’s arrest was “appalling” and called for his immediate release.
“Rather than persecuting Mr Navalny, Russia should explain how a chemical weapon came to be used on Russian soil,” he said.
What happened in the lead-up to the arrest?
When Mr 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,” Mr 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, Mr 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.
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.