Kremlin critic Alexei Navalny has been detained after flying back to Moscow five months after he was nearly killed by a nerve agent attack last year.
Mr Navalny, 44, was seen being led away by police at passport control.
His flight from Berlin was diverted from one of Moscow’s airports to another after crowds gathered there.
The activist says the authorities were behind the attempt on his life, an allegation backed up by investigative journalists but denied by the Kremlin.
“I know that I’m right. I fear nothing,” Mr Navalny told his supporters and the media at Moscow’s Sheremetyevo airport just minutes before his detention.
He added that all criminal cases against him had been “fabricated”.
Earlier on Sunday, metal barriers were erected inside Moscow’s Vnukovo airport, where the plane was originally scheduled to land, and Russian media reported that several activists – including key Navalny ally Lyubov Sobol – were detained there.
Mr Navalny’s spokeswoman Kira Yarmysh posted on social media pictures of police cars at the airport.
Mr Navalny – who had been treated in Germany – earlier urged supporters to meet him off the flight, and a “Let’s meet Navalny” page was set up on Facebook (in Russian). Thousands of people said they would go or expressed an interest, despite forecasts of extreme cold and the ongoing Covid-19 pandemic.
Mr Navalny collapsed on an internal flight in Siberia last August, and it later emerged he had been poisoned with a Novichok nerve agent.
Russian authorities have consistently denied any role in the poisoning, and the Kremlin has rejected Mr Navalny’s claims that President Vladimir Putin himself ordered it.
The Putin critic has said he misses Moscow, is almost fully recovered from the attack, and that there was never any doubt he would return.
The Russian authorities earlier warned Mr Navalny could face imprisonment after missing a prison service deadline in December to report at an office in Moscow.
The prison service accuses him of violating conditions imposed after a conviction for embezzlement, for which he received a suspended sentence. He has always condemned the case as politically motivated.
Separately, Russia’s investigative committee has launched a new criminal case against him on fraud charges related to transfers of money to various NGOs, including his Anti-Corruption Foundation.
Mr Navalny has asserted that Mr Putin is doing all he can to stop his opponent from coming back by fabricating new cases against him.
News media from around the world gathered at Berlin airport to record the activist’s departure from Germany – but Russian federal TV channels and news agencies are ignoring his return.
In August, the opposition leader collapsed on a plane flying home from Tomsk in Siberia to Moscow and the pilot diverted the flight to the city of Omsk, from where he was eventually allowed to fly on to Germany in an induced coma.
He was released from hospital in Berlin in September to continue his recuperation.
Mr Navalny said recently he was able to do push-ups and squat exercises, and therefore had probably almost fully recovered.
Last month, investigative reporters named three FSB agents who had travelled to Tomsk at the time Mr Navalny was there, and said the specialist unit had tailed him for years.
Mr Navalny then, in a phone call, duped an FSB agent named Konstantin Kudryavtsev into revealing details of the operation against him, according to the Bellingcat investigative group.
The agent told him that the Novichok used to poison him was placed in his underpants.
Mr Kudryavtsev said during the phone call he had been sent to Omsk later to seize Mr Navalny’s clothes and remove all traces of Novichok from them.
President Putin has dismissed the investigation by Bellingcat and others into who poisoned Mr Navalny as “a trick” and said that he was backed by US intelligence services.