An ambulance was called for Navalny, his spokeswoman says, as concerns over his health grow.

Aleksei A. Navalny, the jailed Russian opposition leader, has been suffering from stomach pain so acute that prison officials were forced to call an ambulance to treat him, his spokeswoman said Thursday.

Mr. Navalny, Russia’s most vocal critic of the Kremlin, is being held in Penal Colony No. 2, in the town of Pokrov just east of Moscow, where he says he has been repeatedly placed in so-called punishment cells for trivial misdemeanors such as washing his face ahead of schedule. He has repeatedly complained about conditions in the prison and about the lack of access to proper medical care.

Concerns about Mr. Navalny’s health have been growing in recent months and have led to rare, public petitions from groups of Russian lawyers, doctors and lawmakers, who — despite the risk of being prosecuted for dissent — have used their full names to demand that he receive better medical care.

Kira Yarmysh, Mr. Navalny’s spokeswoman, said that an ambulance had to be called early in the morning on Saturday because he was experiencing acute stomach pain.

“The ambulance was called by the penal colony’s personnel,” Ms. Yarmysh said on Thursday in a response to written questions. “We understand that the situation must have been critical.”

There has been no update on Mr. Navalny’s condition since then, Ms. Yarmysh said, because “it has been very difficult to get in touch with him.” Mr. Navalny communicates with the world through his lawyers.

In a series of posts on Twitter on Tuesday, Ms. Yarmysh also said that Mr. Navalny had lost more than 17 pounds after spending over 15 days in a punishment cell. After being released from the cell last week, Mr. Navalny was sent back to it on Monday, Ms. Yarmysh said — his 13th time in the cell since he was imprisoned in March last year.

In January 2021, Mr. Navalny returned to Russia from Germany, where he had been recovering from being poisoned by a military-grade nerve agent. Since then, Mr. Navalny has been sentenced twice by Russian courts — to nine years in prison overall — for ostensibly violating his parole as well as embezzling donations from his supporters. The sentences are widely regarded as a politically motivated campaign to silence Mr. Navalny after Russian security agents failed to poison him. The Kremlin has denied involvement in the poisoning.

Mr. Navalny became Russia’s most prominent opposition leader by exposing high-level corruption and by challenging President Vladimir V. Putin and his party, United Russia. In 2013, he received more than 600,000 votes in the election for mayor of Moscow, coming close to sending the Kremlin-picked candidate into a second round of voting and showing that the opposition enjoyed considerable support, at least in the Russian capital.