The 44-year-old foe of Russia's President Vladimir Putin felt unwell on a flight back to Moscow from Tomsk, a city in Siberia, Kira Yarmysh said on Twitter. “The plane made an emergency landing in Omsk. Alexei has a toxic poisoning,” Yarmysh tweeted. 

She told the Echo Moskvy radio station that Navalny was sweating and asked her to talk to him so that he could “focus on a sound of a voice." He then went to the bathroom and lost consciousness. 

Yarmysh says the politician must have consumed something from the tea he drank this morning. 

“Doctors are saying the toxin was absorbed quicker with hot liquid,” she tweeted, adding that Navalny’s team called police to the hospital. 

The politician is in grave condition, Russia's state news agency Tass reported, citing the chief doctor at the hospital.

Last year, Navalny was rushed to a hospital from prison where he was serving a sentence following an administrative arrest, with what his team said was suspected poisoning. Doctors then said he had a severe allergic attack and discharged him back to prison the following day.

Navalny's Foundation for Fighting Corruption has been exposing graft among government officials, including some at the highest level. Last month, the politician had to shut the foundation after a financially devastating lawsuit from Yevgeny Prigozhin, a businessman with close ties to the Kremlin. 

The most prominent member of Russia's opposition, Navalny campaigned to challenge Putin in the 2018 presidential election, but was barred from running. 

He set up a network of campaign offices across Russia and has since been putting forward opposition candidates in regional elections, challenging members of Russia's ruling party, United Russia. 

Like many other opposition politicians in Russia, Navalny has been frequently detained by law enforcement and harassed by pro-Kremlin groups. In 2017, he was attacked by several men who threw antiseptic in his face, damaging one eye.

Also read: Russia begins production of COVID-19 vaccine