An ambulance was called early Saturday because of acute stomach pains but Navalny received no diagnosis, one of his lawyers, Vadim Kobzev, wrote on Twitter after visiting him in prison.

"We believe he is slowly being administered low doses of poison" in pills he is given without identification, Veduta said.

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A video documentary about Navalny won an Oscar last month. The documentary portrays Navalny's career of fighting official corruption, his near-fatal poisoning with a nerve agent in 2020 that he blames on the Kremlin, his five-month recuperation in Germany and his 2021 return to Moscow, where he was taken into custody. He was later sentenced to 2 1/2 years in prison and last year was convicted of other charges and given another nine-year term.

Navalny has faced unrelenting pressure from Russian authorities, and has been in and out of isolation in a tiny punishment cell. He is allowed to write letters or have his lawyers visit occasionally.