Frail Dr Dmitry Kolker, 54, was hauled from his hospital bed in Novosibirsk, Russia, and flown four hours to Moscow where he is now incarcerated in notorious Lefortovo prison.
He is charged with collaborating with Chinese intelligence agencies, a potentially acutely embarrassing blow to Putin who has boasted of his close ties with Beijing.
Dr Kolker heads Novosibirsk State University’s quantum optical technologies laboratory, and is regarded as a world expert on lasers, having previously collaborated with the Rutherford Appleton Laboratory in Oxford major institutes in many countries.
It is not clear if his alleged “treason” accusation is related to advanced military secrets concerning new-age weapons.
He and his family deny all accusations against Dr Kolker, the details of which remain secret, and they accuse the authorities of “torturing” a dying man.
He is now locked inside Moscow’s notorious Lefortovo prison. His apartment was searched and equipment seized.
Colleagues fear a “Stalinist witch-hunt” against the “dying scientist”, amid a rising number of arrests of academics under suspicion of espionage seen as a return to sinister Soviet ways.
A tube was ripped from his vein and he was taken away into custody despite being too ill to undergo chemotherapy, say his relatives, who claim he has just days to live.
His daughter Alina Mironova, 29, said: “Our family is going through total hell.
“Our father cannot pass away surrounded by his family members.
“He has Stage 4 pancreatic cancer, which is a death sentence.
“But to face this sentence away from home, at a detention centre without proper medical help, is the worst that could happen to a man.”
She said he was “a true patriot of his country” but he is now held incommunicado.
“He obtained his knowledge and learned his skills abroad, but brought them back here, to Russia.
“He is a doctor of sciences, head of laboratory, and a talented scientist.”
The distraught daughter said: “He can’t feed himself independently, his body doesn’t take the food in.
“Chemotherapy cannot help anymore, he was prescribed palliative care.
“We don’t even know how he feels. Is he alive now, will we see him again?”
She said he was grabbed from hospital on Friday by the FSB security service, which Putin once headed.
“We can only pray he is given medical help, and that we, his family, will see him again to give him a decent farewell.”
The scientist’s son Maxim, 22, admitted his father had travelled to China on a lecturing trip. But he was accompanied all the time by an FSB agent who approved every word of his lectures, he said.
The furious son said Dr Kolker’s arrest from his hospital bed was “utter chaos”.
“Within hours he was taken out of hospital, put on board a secret plane - and vanished,” he said.
“I don’t any longer care what kind of investigation they will carry out, or about his freedom. I want my father to peacefully die in good conditions, with relatives he needs for help and support.”
Alina said her father was known all over the world, and he had close links with laboratories in France and Germany.
“I am not afraid to defend our rights,” she said.
“Our family is convinced that the ban on communication with this terminally-ill man, the failure to provide information about the state of his health, the denial of basic assistance and the presence of relatives in the last days of life is a violation of human rights.
“And also a sophisticated form of torture. A person deserves a right to die at home.
“You can just look at these pictures [of the frail scientist] and assess his condition.
“They were taken on the day he was taken to the clinic because his condition worsened.”
Pictures showed the scientist’s mother - Alina’s grandmother - taking care of him, she said.
Alina posted: “Just. Look. At. These. Pictures.
“He cannot walk, or eat, or take care of himself. He needs special food and care, in the circle of his family and friends.”
Dr Kolker was a talented pianist and organist.