TALLINN, Estonia >> When Alexei Navalny turns 47 on Sunday, he’ll wake up in a bare concrete cell with hardly any natural light.

He won’t be able to see or talk to any of his loved ones. Phone calls and visits are banned for those in “punishment isolation” cells, a 2-by-3-meter (6 1/2-by-10-foot) space. Guards usually blast patriotic songs and speeches by President Vladimir Putin at him.

“Guess who is the champion of listening to Putin’s speeches? Who listens to them for hours and falls asleep to them?” Navalny said recently in a typically sardonic social media post via his attorneys from Penal Colony No. 6 in the Vladimir region east of Moscow.

He is serving a nine-year term due to end in 2030 on charges widely seen as trumped up, and is facing another trial on new charges that could keep him locked up for another two decades. Rallies have been called for Sunday in Russia to support him.

Navalny has become Russia’s most famous political prisoner — and not just because of his prominence as Putin’s fiercest political foe, his poisoning that he blames on the Kremlin, and his being the subject of an Oscar-winning documentary.

He has chronicled his arbitrary placement in isolation, where he has spent almost six months. He’s on a meager prison diet, restricted on how much time he can spend writing letters and forced at times to live with a cellmate with poor personal hygiene, making life even more miserable.

Most of the attention goes to Navalny and other high-profile figures like Vladimir Kara-Murza, who was sentenced last month to 25 years on treason charges. But there’s a growing number of less-famous prisoners who are serving time in similarly harsh conditions.

Memorial, Russia’s oldest and most prominent human rights organization and a 2022 Nobel Peace Prize laureate, counted 558 political prisoners in the country as of April — more than three times the figure than in 2018, when it listed 183.

The Soviet Union’s far-flung gulag system of prison camps provided inmate labor to develop industries such as mining and logging. While conditions vary among modern-day penal colonies, Russian law still permits prisoners to work on jobs like sewing uniforms for soldiers.

In a 2021 report, the U.S. State Department said conditions in Russian prisons and detention centers “were often harsh and life threatening. Overcrowding, abuse by guards and inmates, limited access to health care, food shortages and inadequate sanitation were common in prisons, penal colonies, and other detention facilities.”

Andrei Pivovarov, an opposition figure sentenced last year to four years in prison, has been in isolation at Penal Colony No. 7 in northern Russia’s Karelia region since January and is likely to stay there the rest of this year, said his partner, Tatyana Usmanova. The institution is notorious for its harsh conditions and reports of torture.

The 41-year-old former head of the pro-democracy group Open Russia spends his days alone in a small cell in a “strict detention” unit, and is not allowed any calls or visits from anyone but his lawyers, Usmanova told The Associated Press. He can get one book from the prison library, can write letters for several hours a day and is permitted 90 minutes outdoors, she said.

Other inmates are prohibited from making eye contact with Pivovarov in the corridors, contributing to his “maximum isolation,” she said.

“It wasn’t enough to sentence him to a real prison term. They are also trying to ruin his life there,” Usmanova added.

Pivovarov was pulled off a Warsaw-bound flight just before takeoff from St. Petersburg in May 2021 and taken to the southern city of Krasnodar. Authorities accused him of engaging with an “undesirable” organization -— a crime since 2015.

Several days before his arrest, Open Russia had disbanded after getting the “undesirable” label.

After his trial in Krasnodar, the St. Petersburg native was convicted and sentenced in July, when Russia’s war in Ukraine and Putin’s sweeping crackdown on dissent were in full swing.

He told AP in a letter from Krasnodar in December that authorities moved him there “to hide me farther away” from his hometown and Moscow. That interview was one of the last Pivovarov was able to give, describing prison life there as “boring and depressing,” with his only diversion being an hour-long walk in a small yard. “Lucky” inmates with cash in their accounts can shop at a prison store once a week for 10 minutes but otherwise must stay in their cells, he wrote.

Letters from supporters lift his spirits, he said. Many people wrote that they used to be uninterested in Russian politics, according to Pivovarov, and “only now are starting to see clearly.”