"The rapid deterioration of his health condition raises our extreme concerns," they said.
The health of Kremlin critic Alexei Navalny is deteriorating as he serves his prison sentence in a Russian, his lawyers have said.
Navalny was sentenced to a two-and-half-year jail term last month for violating the terms of his probation as he was recovering from a nerve agent attack in Germany.
He is serving the sentence at a penal colony in Pokrov, about 85 kilometres east of Moscow, known for its harsh detention conditions. Earlier this month, Navalny described it as a "concentration camp".
In a tweet, one of Navalny's lawyers said that he "feared for his life and his health."
Earlier, Navalny's lawyers said in a statement released on Wednesday and a Facebook post by his close ally Leonid Volkov that since the end of last week the opposition figure has been experiencing "severe back pain", "cannot stand on his foot" and has fainted.
"The rapid deterioration of his health condition raises our extreme concerns."
They added that he had "received two ibuprofen pills throughout the entire time".
"We believe that he may be in a prison hospital now, and the IK-2 administration may be trying to hide that fact," they added.
The lawyers claimed they have been prevented from seeing Navalny.
The Russian Federal Prison Service said on Thursday that his health is "satisfactory".
Navalny, 44, was arrested on January 17 when he returned to Russia following a five-month stay in Berlin, Germany, where he was treated for Novichok poisoning. He has blamed the nerve agent attack against him on President Vladimir Putin.
The Kremlin vehemently denies this.
His arrest prompted mass demonstrations across Russia in late January and early February at which the authorities detained thousands of protesters.
A new campaign launched earlier this week calling for his release has said a date for a new protest will be set when 500,000 signatures have been gathered. More than 233,000 people have already backed the petition.