The anti-corruption campaigner fell ill during a flight and the plane made an emergency landing in Omsk, Kira Yarmysh said, adding that they suspected something had been mixed into his tea.
The Kremlin said that it wished Mr Navalny a “speedy recovery”.
Mr Navalny, 44, has been a staunch critic of President Vladimir Putin.
Kira Yarmysh, the press secretary for the Anti-Corruption Foundation, which Mr Navalny founded in 2011, tweeted: “This morning Navalny was returning to Moscow from Tomsk.
“During the flight, he felt ill. The plane made an urgent landing in Omsk. Alexei has toxic poisoning.”
She added: “We suspect that Alexei was poisoned by something mixed into [his] tea. It was the only thing he drank since morning.
“Doctors are saying that the toxic agent absorbed faster through the hot liquid. Right now Alexei is unconscious.”
Yarmysh later tweeted that Mr Navalny was on a ventilator and in a coma, and that the hospital was now full of police officers.
She also said that doctors were initially ready to share any information but then they later claimed the toxicology tests had been delayed and were “clearly playing for time, and not saying what they know”.
She has been told the diagnosis will be “towards evening”.
Both Mr Navalny’s wife, Yulia Navalnaya, and doctor, Anastasia Vasilyeva, had arrived at the hospital, Ms Yarmysh said.
She said Mrs Navalnaya was initially denied access to her husband because authorities said the patient had not agreed to the visit she but had now been allowed on to the ward. Dr Vasilyeva had not been allowed on to the ward, Ms Yarmysh said.
She said they had been told that all of Mr Navalny’s belongings would be confiscated, but Mrs Navalnaya did not allow them to be seized and took them with her.
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