ALEKSEI NAVALNY HOSPITALIZED IN RUSSIA IN SUSPECTED POISONING

MOSCOW — Groaning in agony from a suspected poisoning before losing consciousness, the Russian opposition leader Aleksei A. Navalny was rushed to a Siberian hospital on Thursday after the plane he was flying on made an emergency landing because of his sudden illness.

© Shamil Zhumatov/Reuters Mr. Navalny, center, is a vocal critic of President Vladimir V. Putin.

Doctors at the No. 1 Clinical Hospital in Omsk, the Siberian city where the plane landed, initially said that Mr. Navalny, a fierce critic of President Vladimir V. Putin, was on a ventilator in “serious condition” but later reported that his condition, though still grave, had stabilized.

© Sergei Chirikov/EPA, via Shutterstock A video posted on social media on Thursday showed Aleksei A. Navalny, Russia’s most prominent opposition leader, being wheeled to an ambulance in Omsk, Russia.

Mr. Putin’s spokesman, Dmitri S. Peskov, told journalists that the Kremlin wished Mr. Navalny a swift recovery, “as we would for any citizen of Russia,” and would, if asked, provide help to get the opposition leader transferred to a hospital abroad.

Mr. Navalny, who often refers to Mr. Putin as the head of “a party of crooks and thieves,” is the latest in a long line of Kremlin opponents to be suddenly afflicted by bizarre and sometimes fatal medical emergencies. The Kremlin and its supporters have for years regarded him as an enemy because of the investigations he has led into graft by officials — including, most vividly, the former prime minister, Dmitri A. Medvedev. Mr. Navalny has been harassed and jailed numerous times for short periods, but the authorities have, until now, refrained from harsher steps that could elevate his national profile.

© Alexey Malgavko/Reuters The hospital in Omsk where Mr. Navalny was on a ventilator in intensive care.

Mr. Navalny’s spokeswoman, Kira Yarmysh, who was traveling with the anticorruption campaigner on a flight destined for Moscow from Tomsk, said on Twitter, “We assume that Aleksei was poisoned with something mixed with his tea.”

Pavel Lebedev, a passenger who posted a picture of Mr. Navalny drinking tea at the airport before departure, said on Instagram that the opposition activist “went to the toilet at the beginning of the flight and didn’t return. He started feeling very bad. They could barely revive him and he’s still crying out in pain.”

Videos posted by Russian news outlets showed an apparently unconscious Mr. Navalny being wheeled on a gurney to an ambulance waiting on the tarmac at the Omsk airport.

The state-owned news agency Tass quoted an unidentified law enforcement source as saying that the authorities were not yet considering the possibility of a deliberate poisoning. It said t