The 44-year-old is said to be in a coma and on a ventilator in a hospital intensive care unit after falling ill on a flight back to Moscow from Tomsk, a city in Siberia.
Arkady Ostrovsky, The Economist magazine’s Russia editor, said the plane made an emergency landing about halfway into the flight, citing Navalny’s press secretary.
The Daily Beast reported that Kira Yarmish, Navalny's press secretary, said he was poisoned with some kind of toxin and was unconscious.
“We assume that Alexei was poisoned with something mixed into his tea. That was the only thing he drank this morning. The doctors say that the toxin was absorbed more quickly because of the hot liquid. Right now Alexei is unconscious,” she said, according to the report.
Yarmish wrote on Twitter that he is "in a coma in grave condition."
Kremlin spokesman Dmitry Peskov said it was necessary to wait for the test results showing what caused Navalny's condition. Although Russia has not fully reopened its borders after a coronavirus lockdown, Peskov said authorities would consider a request to allow Navalny to leave the country for treatment.
Yarmish told the Echo Moskvy radio station that during the flight 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.
Navalny's wife arrived at the hospital Thursday but could not immediately see him. His spokeswoman said doctors have not yet given permission for Navalny to have visitors. Doctors said he is stable but remains in a coma.
Reuters reported that at least one of the doctors at the hospital said it was not certain that he had been poisoned and “natural poisoning” was being considered.
Yarmish on Twitter bristled at that suggestion: “Of course. It's just the tea was bad. This is what the state propaganda is going to do now — yell that there was no deliberate poisoning, he (did something) accidentally, he (did something) himself.”
The widow of Alexander Litvinenko, the Russian agent who was killed in London by radioactive poisoning in 2006, voiced concerns that Navalny’s enemies within Russia may have decided that it’s time to use a “new tactic.”
“It was obvious he would not be stopped,” Marina Litvinenko told The Associated Press from Sicily, Italy. “Maybe they decided to do a new tactic not to stop him just with an arrest but to stop him with poison. It looks like a new tactic against Navalny.”
British Foreign Secretary Dominic Raab said in a tweet that he was “deeply concerned” by the reports about the suspected poisoning of the politician.
Speaking at the White House, President Trump said U.S. officials were looking at the situation surrounding the opposition leader, Reuters reported.
Russian authorities have recently opened a criminal case against Navalny for allegedly slandering a veteran of World War II. The case announced Monday by the Investigative Committee stems from comments Navalny posted on social media about a video on Kremlin-backed RT television.
In it, several prominent Russians speak in favor of the July 1 referendum on constitutional reform that would allow Putin to run for another two terms. Among those featured was Ignat Artemenko, a 93-year-old WWII veteran.
Navalny’s post criticized the people in the video as being “without conscience.” The case against Navalny says the comments were “false information discrediting the honor and dignity of a veteran of the Great Patriotic War.”
If convicted, Navalny could face a fine of up to one year’s income or 240 hours of community service.
Vyacheslav Gimadi, a lawyer with Navalny's foundation, said the team is requesting Russia's Investigative Committee open a criminal probe.
“There is no doubt that Navalny was poisoned because of his political stance and activity,” Gimadi said in a tweet on Thursday
Fox News' Amy Kellogg and Stephen Sorace, along with The Associated Press, contributed to this report