Russian doctors were battling to save the life of leading opposition figure Alexei Navalny on Thursday after he was rushed to intensive care in Siberia suffering from what his spokeswoman said was a suspected poisoning.
Navalny, a 44-year-old lawyer and anti-corruption campaigner who is among President Vladimir Putin's fiercest critics, was hospitalised in the city of Omsk after he lost consciousness on a flight and his plane made an emergency landing.
"Doctors aren't just doing everything possible. The doctors are really working now on saving his life," the hospital's deputy head doctor Anatoly Kalinichenko told journalists in Omsk.
Navalny's spokeswoman Kira Yarmysh said he was on a ventilator in a coma and his condition was serious but stable.
"Alexei has toxic poisoning," Yarmysh wrote on Twitter, describing how he was taken ill during the flight from the city of Tomsk to Moscow.
The hospital has not given any diagnosis while the regional health ministry said Navalny was in a natural, not induced, coma.
His team said the hospital was ill-equipped and his doctor Anastasia Vasilyeva said she had asked for the Kremlin's help to transfer him to a European clinic.
Putin's spokesman Dmitry Peskov wished Navalny a "speedy recovery" adding that the Kremlin would help move him abroad if needed.
Peskov said claims of poisoning were "only assumptions" until tests proved otherwise.
Yarmysh claimed Putin was responsible for poisoning Navalny, saying: "Whether or not he gave the order personally, the blame lies with him."