Aleksei A. Navalny, Russia’s most prominent opposition leader, published a letter on Tuesday describing an arduous transfer to his new penal colony in the Arctic, the first time his supporters had heard from him in three weeks.

Mr. Navalny’s comments, posted on his social network accounts and written with a heavy dose of irony and humor, highlighted his good spirits and seemed intended to assuage concerns among allies who had grown anxious about his health and status since his sudden disappearance from the public eye on Dec. 5.

“I am your new Father Frost,” Mr. Navalny wrote, referring to the Russian version of Santa Claus. “I have a sheepskin coat, a hat with earflaps; I should get felt boots soon, and I have grown a beard during the 20-day transit,” he said.

“But the main thing is that I know live above the Arctic Circle.”

Mr. Navalny, 47, is a longtime antagonist of President Vladimir V. Putin who has been subject to increasingly harsh punishment over the past year. His transfer to one of Russia’s high-security “special regime” penal colonies had been expected since September, when he lost an appeal against the 19-year sentence he is serving.

But his lawyers and allies were not notified in advance that he would be moved, raising fears and speculations about his health after legal team was unable to contact him.

Mr. Navalny has been in custody since his detention in January 2021 at a Moscow airport, where he had arrived after spending months in Germany recovering from poisoning by a nerve agent. Mr. Navalny and Western governments have accused the Kremlin of the poisoning, a charge that Russian officials have denied.