Russian opposition leader Alexei Navalny
was detained at Moscow’s Sheremetyevo airport on his return from treatment in Germany, after a suspected poisoning attempt on his life by Russia’s FSB spy agency.
The activist, 44, was led away by police at passport control.
Thousands had gathered at a different Moscow airport to greet his flight from Berlin, but the plane was diverted.
News of Mr Navalny's detention prompted condemnation from the European Union, France and Italy, which called for his immediate release.
US President-elect Joe Biden's incoming national security adviser, Jake Sullivan echoed the call.
"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," he said.
Navalny’s spokesperson, Kira Yarmysh, said the Kremlin was terrified of Navalny and of images showing large crowds wanting to greet him.
“Until recently, it was impossible to believe they [the authorities] were so scared. But here’s the confirmation,” she tweeted.
After landing in Russia, Navalny and his wife left the plane before taking the airport bus with other passengers towards the terminal building.
Mr Navalny said he was “very happy to be back”.
“This is the best day in the last five months. I’m home," he said.
He said the criminal cases against him are all “fabricated”, adding that justice and truth were on his side.
The Moscow department of Russia’s federal penitentiary service confirmed that Navalny had been arrested.
The official cause was failure to appear at a parole hearing. He could face years behind bars if a suspended sentence he received in 2014 is amended to a prison term.
Separately, Russian prosecutors have launched a new criminal case against Mr Navalny on fraud charges related to transfers of money to various charities, including his Anti-Corruption Foundation.
The activist earlier argued that Mr Putin was doing all he could to stop him from returning to Russia by fabricating new cases against him.
Poisoned with Novichok
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.
European experts later said he had been poisoned with the extremely toxic chemical weapon Novichok, developed by Soviet scientists in the Cold War.
He was released from a hospital in Berlin in September to continue his recuperation.
Russian law enforcement had threatened to jail Navalny in a bid to force him into exile in Berlin but Navalny, said: “I have every right to come back.”