Alexei Navalny's friend, Bristol mayor Marvin Rees, says he is "horrified" about what has happened to the "brave" outspoken Kremlin critic and is concerned about his family.
The Russian opposition leader was sentenced to two years and eight months in jail earlier this month for violating parole in a case he said was politically motivated.
He has accused the Kremlin of deliberately targeting him - accusations denied by Russian officials.
Speaking to Sky News, Mr Rees, who first became friends with him a decade ago when they did a fellowship programme for several months at Yale University in the US, said what had happened to Mr Navalny was "absolutely appalling".
He said his friend "has committed his life to exposing fraud in Russia that's taken money out of the pockets of millions of people and put it into the hands of a few well-positioned people at the top.
"I'm horrified about what's happening to him. I'm very concerned about his family."
Mr Rees said while at Yale, Mr Navalny had uncovered a multi-billion dollar fraud about an oil pipeline in Siberia.
He added that other fellows questioned the wisdom of Mr Navalny returning to Russia and that he may not be safe.
The Bristol mayor said: "Alexei's point was, if I don't go back, they are going to say I'm a puppet of the West. They're going to say I've been at Yale and I'm associated with the CIA. They're going to spread all these rumours.
"So he went back in the face of all the warnings from his fellow world fellows about the threats to his own and his family's safety. And you just have to admire that, a brave man who knew what was coming."
He added: "What's happened is exactly what he said would happen. I just hope people can see through it for what it is."
At Yale, Mr Navalny was a "very considerate person", "sincere and committed" and drove Mr Rees to the supermarket and other places as he did not have a car.
Since Mr Navalny was detained early this year, there have been large anti-Kremlin protests across Russia calling for him to be released, and many of his supporters have been detained.
Asked what would happen next, Mr Rees said: "I've no idea how this is going to go.
"But I think one thing is for sure, the integrity and commitment and bravery of Alexei is being shown on the world stage and the opposite spirit of some of those who have been trying to kill him is also being put on show - and it's up to the Russian people to judge."
Mr Rees said that as his friend was recovering from being poisoned last summer, Mr Navalny sent him and other fellows an email thanking them for their support.
The Russian also sent him a text message back in 2016 wishing him good luck in his campaign to be Bristol mayor, and Mr Rees is looking forward to one day showing Mr Navalny around the British city.
"That's the promise. That's the tour I hope I get to give," said Mr Rees.