By PATRICK REEVELL, ABC News

(MOSCOW) — Lawyers for the Russian opposition leader Alexey Navalny said they are concerned about his health in prison and have accused authorities of blocking access to him.

Navalny, known as President Vladimir Putin’s fiercest critic, arrived in a penal colony about 60 miles east of Moscow earlier this month, where he has been sent to serve a two-and-a-half year prison sentence he received in a trial widely criticized as political.

Lawyers for Navalny on Wednesday said they had tried to make a scheduled visit to him at the prison, Correctional Colony No 2, but were stopped from seeing him by prison authorities, whom they accused of hiding Navalny. The lawyers, Olga Mikhailova and Vadim Kobzev, said they were worried Navalny’s health was deteriorating in prison.

They said Navalny had been suffering back pain for several days and that one leg had now gone numb and he was unable to stand on it. He had been seen by a doctor last Friday but had since been denied any treatment, besides being given two ibuprofen tablets, the lawyers said.

Navalny chose to return to Russia after narrowly surviving a nerve agent poisoning last summer that has been linked to the Russian security services. His supporters fear he could still face fresh attempts on his life in prison.

“In the circumstances that we are all aware of, the sharp worsening of his well-being cannot but cause extreme concern,” the two lawyers wrote in a statement.

Following the reports, Russia’s Federal Penitentiary Service on Thursday announced it had conducted a medical examination of Navalny at the prison and that his health was “considered stable, satisfactory.”

The prison where Navalny is being held in the Vladimirskaya region is notorious among Russian prisoners for its strictness and for frequently blocking access to inmates’ lawyers and relatives. Prisoners spend much of the day on their feet and have virtually no free time, former inmates have told ABC News and other media.

Denying inmates adequate medical care is also routine in Russian prisons, according to human rights monitors. Prisoners can go weeks requesting medical examinations before seeing a doctor and treatment is often limited to ibuprofen tablets.

Navalny spent months recuperating in Germany following his near fatal poisoning with a nerve agent last August that left him in a coma. Navalny, 41, had to relearn how to move, undergoing physical therapy in Germany, but he has appeared to recover well. His doctors, however, said the Novichok nerve agent could leave lingering nerve damage and other health problems.

In recent weeks Navalny himself has posted upbeat messages from prison via his lawyers. In messages from the penal colony this week, he compared inmates being forced to line up each morning to that of Stormtroopers in the “Star Wars” films.

Navalny’s team recently announced plans for a new street protest later this spring. Tens of thousands of people joined protests across Russia in late January after Navalny was arrested following his return to Moscow. But after two weeks of intense crackdown that saw thousands detained, Navalny’s organizers called off any further street demonstrations, saying the movement needed to conserve its strength and that it would be irresponsible to continue when it was clear short-term demonstrations would not force the Kremlin to release Navalny.

Although the protests were unusually large for Russia, they were easily dispersed by authorities with riot police. On the day of Navalny’s jailing, only a couple of thousand people demonstrated in Moscow.

Navalny’s team said they were taking a different approach and would not name a date for the new protest until around 500,000 people had said they would attend by registering at a website created by Navalny’s group. After more than two days, the website shows nearly 247,000 people have registered to take part in the protest.

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