Renowned biophysicist Maksim Frank-Kamenetsky left his native Russia 21 years ago to settle in the United States.

Last fall, he returned to Moscow on a four-month teaching assignment.

But instead of the homecoming he had looked forward to, his stay in Russia left him so upset that he decided to pen an outspoken letter to Russians.

"During my stay here, an overwhelming feeling of nausea gradually came over me and grew stronger all the time," he wrote in his letter, which has been widely circulated since it was posted online earlier this month. "There has not been such centralization, such concentration of power in this vast country in the hands of one person since Stalin's time."

Frank-Kamenetsky, a professor of biomedical engineering at Boston University famous for his contribution in the field of DNA topology, says he was shocked to witness what he describes as a "rabid" adulation for Russian President Vladimir Putin.

"It's a bona fide cult of personality, like the one I witnessed as a child," he tells RFE/RL's Russian Service.

'Idiotic Mantra'

Such wake-up calls have become rare in Russia, where Western criticism of the country's actions in Ukraine have shored up public support for the Kremlin and its increasingly defiant, isolationist stance.

In his letter, Kamenetsky laments the "idiotic mantra" that he says is being trumpeted by most Russians, including intellectuals, consisting of professing unconditional love for Russia and all things Russian.

He says he was also struck by the rampant anti-Americanism among Russians, a trend he attributes to an enduring postimperial complex caused by the Soviet Union's collapse. Frank-Kamenetsky recalls his consternation upon finding racist T-shirts lampooning U.S. President Barack Obama on sale in Moscow's most famous street, the Arbat, alongside T-shirts extolling Putin as "The Best President."

"Almost everyone is infected, to various degrees, by this anti-Americanism," he says. "Surprisingly, even those who understand the inadmissibility of what Putin is doing in Ukraine don't miss an opportunity to recall the mistakes of U.S. administrations -- the bombing of Yugoslavia, the invasions of Afghanistan and Iraq, and so on and so forth -- and say that America's crimes are much worse and more dangerous that Putin's mistakes."

In his letter, Frank-Kamenetsky takes issue with Russia's annexation of Crimea from Ukraine last March, a move welcomed by a majority of Russians -- 73 percent, according to a September poll by the independent Levada Center.

He calls the annexation a "dangerous precedent" and points out that even Nazi leader Adolf Hitler held talks with France and Great Britain before annexing the German-speaking Sudetenland from Czechoslovakia.

"What about the so-called Kaliningrad region, i.e. Koenigsberg and East Prussia?" the letter continues. "Is this also Russian soil? Or is it German?"

'A Horrible End Or Endless Horror'

Frank-Kamenetsky, who regularly travels to Russia, says his departure from the country this time filled him with unprecedented relief. He says Russia, with its increasingly palpable atmosphere of uncertainty and fear, is sliding back to Soviet ways.

"This fear is well-founded because no one knows what the next step is going to be," he says. "One person is effectively in charge. This is also very scary since, in the light of current events, his ability to make appropriate decisions is highly questionable."

According to Frank-Kamenetsky, a new law requiring Russians with dual nationality to register with Russian migration authorities has also stoked fear in Russians living abroad.

The biophysicist, who holds both Russian and U.S. citizenship, chose to voluntarily register with Russian authorities. But he says many of his Russian colleagues and friends in the United States have resolved not to return to Russia.

"If people don't register within 60 days after entering the country, they risk administrative or criminal prosecution," he says. "This is why many people got really scared."

Fresh from his four-month stint in Russia, Frank-Kamenetsky is not optimistic about the country's prospects. He believes that unless Moscow changes course, Russia's political isolation, combined with the rapid slide of the ruble, spell disaster.

"If a full-blown crisis breaks out, there will be two options: a horrible end [for the regime] or endless horror [for the Russian people]," he predicts. "I think there is no need to say which I would prefer."