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.
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 Mr 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 Mr 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 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.
BBC