The Kremlin may be slowly killing Alexei Navalny in prison. The world must not let it happen.

There are many ways to kill an opponent. Sometimes it’s done out in the open, for the whole world to see — as happened with Russian opposition leader Boris Nemtsov, who was gunned down in plain sight of the Kremlin by an Interior Ministry officer. Often it’s done with techniques that aim to maintain plausible deniability, such as poisoning — a favored method practiced by Soviet and Russian security services against opponents at home and abroad.

Sometimes there are near-misses, as with the author of this article, and, most prominently, with anti-corruption crusader Alexei Navalny, who last year survived a chemical weapons poisoning linked to Russia’s Federal Security Service. After recuperating abroad, Navalny — currently Russian President Vladimir Putin’s most prominent adversary — returned to Russia in January, only to be imprisoned by the authorities under a false charge that has long been debunked by the European Court of Human Rights.

Now the Kremlin is trying to kill him again — this time slowly, painfully and in the confinement of the maximum-security Pokrov IK-2 penal colony some 50 miles east of Moscow.

Navalny is usually not one to complain. Whatever happens to him — be it his multiple arrests, an acid attack on his eyes that nearly cost him his vision or last year’s near-fatal poisoning — he takes it with a stiff upper lip and in good humor. So when he began to experience acute back pains about a month ago, while still in the pretrial detention center, he kept quiet. But he stood throughout his trial because it was too painful for him to sit down.

Alarm bells went off last week when authorities prevented Navalny’s lawyers Olga Mikhailova and Vadim Kobzev from seeing him in the Pokrov colony. When they finally gained access to their client, they saw a man hardly able to walk, struggling to get off his bed, unable to feel his right leg, and suffering sharp and constant back pain. “Navalny forbade us from making this public, but … we decided to come out into the open,” wrote Kobzev. “They are deliberately turning him into a cripple,” Mikhailova added.

It is difficult to find another description. The prison authorities are not only denying civilian doctors access to Navalny. They are also refusing to accept medicines for back pain — or even a handwritten doctor’s note with exercises. Since he is not being seen by doctors, the diagnosis is unknown — but likely causes include a pinched nerve from being held in cramped paddy wagons and prison cells and the lasting effects of last year’s poisoning. In addition, Navalny’s jailers have labeled him a “flight risk” (this for a man who voluntarily returned to Russia even though he was facing arrest). That means that he is woken up every hour at night by prison guards, in what amounts to torture by sleep deprivation.

The mistreatment of Navalny brings horrible memories of past political prisoners in the Soviet Union and Russia who perished in the country’s modern-day gulag system because of deliberate denial of medical care.

Navalny is not only a political prisoner, as has been recognized by the Council of Europe and by Memorial, Russia’s leading human rights group. (He is among 377 individuals currently on its list.) He is also a personal prisoner of Putin. His “crime” consists of having the tenacity to create a political opposition in a country where one isn’t supposed to exist. In a system where the state controls everything — from television networks to ballot access to party registration — he has formed a popular (and overwhelmingly young) movement that challenges Putin’s authority, undermines the claim that there is no alternative to his rule, and punches the regime in its weakest spot: corruption.

“Navalny is becoming … the leader of public discontent with the regime, and this erodes Putin’s legitimacy,” Lev Gudkov, director of the Levada Center, Russia’s sole independent pollster, recently noted. Putin’s plummeting public confidence numbers (now down in the 20s), the pitiful 27 percent support for his ruling party six months before the parliamentary election and the mass nationwide protests over Navalny’s arrest in January all attest to this.

Putin’s response is now being carried out by Navalny’s jailers — complete with sleep deprivation, constant pain and denial of medical care, which several hundred Russian doctors described in an open letter last week as “deliberate torture.” “What is happening here is personal revenge,” wrote Yulia Navalnaya, the opposition leader’s wife. “This must be stopped immediately.”

Indeed, it must — and only one thing can stop it: direct personal intervention by the leaders of Western democracies to the man who is behind it. Torture and mistreatment are unacceptable in any case, even for actual criminals — let alone for a man who is imprisoned for his political beliefs. It’s a violation of every treaty and convention Russia has signed up to.

It is a matter of historical record that personal advocacy by Western leaders has helped save the lives (and freedom) of many political prisoners, both in Soviet times and in recent years. President Biden has made headlines by stating the truth when he described Putin as a “killer.” It’s now in his power to pick up the phone and ensure that the list of victims does not grow by another name.