"I demand that a doctor be allowed to see me, and until this happens, I am declaring a hunger strike," he said in the letter.
The whereabouts of Navalny, 44, had been a mystery since he left jail in Moscow.
Alexander Klyachin, the owner of Russia’s largest international chain of hotels, began his stint this month at the helm of what is widely considered to be Russian Jewry’s most visible institution
He is due to spend a little over 2-1/2 years behind bars, according to his lawyers, one of whom, Vadim Kobzev said on Twitter he had been moved from his Moscow jail.
Currently, the only regular flights to Israel fly from Frankfurt or New York, after the government shut down all commercial flights into Israel on January 25.
Navalny's allies called on their supporters to immediately protest against the ruling in central Moscow.
The high-ranking delegation invited to Moscow was headed by Samir al-Mashharawi, one of the founders of Dahlan’s Fatah Democratic Reform group.
Navalny called on his supporters to protest after being arrested last weekend when he returned to Moscow for the first time after being poisoned in August.
“Over 300 buildings have been reconstructed using construction materials provided by Russia as part of humanitarian work underway to reconstruct 450 buildings.”