Kremlin critic Alexei Navalny has taken to social media to tell his supporters not to worry about him.
Reappearing for the first time since 6 December, he confirmed his arrival at an Arctic prison and said he was in good spirits.
His team had had no contact with him while he was being moved from another jail to the east of Moscow.
Considered Vladimir Putin’s most vocal opponent, Mr Navalny has been imprisoned since 2021.
Writing on X, formerly known as Twitter, that he was “the new Santa”, Mr Navalny confirmed on Tuesday that he had been moved to the IK-3 penal colony, nicknamed “Polar Wolf”, in the northern town of Kharp, some 1,900km (1,200 miles) northeast of Moscow.
IK-3, in the Yamalo-Nenets autonomous district, is seen as one of the toughest jails in Russia and most detainees held there have been convicted of serious crimes.
Mr Navalny said he had been transported with “such precaution” and along “such a strange route” that he was surprised when he was told that his lawyer was there to see him, as he did not think he would be found before “mid-January”.
“He told me that you had lost me, and some of you were even worried. Thanks very much for your support!” he wrote. “Don’t worry about me. I’m fine.”
But Mr Navalny’s chief of staff, Leonid Volkov, told the BBC that concern for the opposition leader remained high:
“We are worried. We won’t forget for a second that he is in custody of the very people who tried to kill him three-and-a-half years ago and that he is Putin’s very personal political prisoner,” Mr Volkov said.
“This forced journey is another proof,” he added.
Mr Navalny had previously been held in Melekhovo, 235 km (145 miles) east of Moscow. He is facing a 19-year jail term, for founding and funding an extremist organisation – accusations he has always rejected as politically motivated.
He is only able to post on social media when he has access to his lawyers and last month three members of his legal team were placed on a list of extremists.
Mr Navalny suggested he had been moved first to the capital, then east to the Urals mountain region and again to the west before being moved north of the Arctic Circle.
“The 20 days of my transportation were pretty exhausting, but I’m still in a good mood, as befits a Santa Claus,” he wrote.
The Kremlin critic continued his tongue-in-cheek message: “Since I’m Santa Claus, you’re probably wondering about the presents. But I am a special-regime Santa Claus, so only those who have behaved very badly get presents.”
The US said that while it welcomed reports that Mr Navalny had been located, it remained “deeply concerned” about his well-being and conditions of detention.
Mr Navalny’s spokeswoman Kira Yarmysh said Russian authorities were intent on isolating him and “trying to make his life as unbearable as possible”.
“This colony is very distant, it is very difficult to access it and for lawyers, it will be very difficult to go there and to see Alexei,” she added.
Mr Navalny’s aide Ivan Zhadov said the change of prison demonstrated how “the system deals with political prisoners, trying to isolate and suppress them”.
His team had grown increasingly worried after he failed to appear at several court hearings.
Mr Navalny made his name as a campaigner against corruption, gathering millions of views for his video investigations.
A charismatic campaigner, he seemed to be the only Russian opposition leader capable of mobilising people in large numbers across Russia to take part in anti-government protests.
But in 2020, he was poisoned in Siberia by what Western laboratories later confirmed to be a nerve agent.
He was treated abroad. On returning to Russia in January 2021, he was immediately arrested.
