Alexei Navalny, a prominent Russian opposition leader and anti-corruption lawyer, is in a Russia hospital after being poisoned with a toxin believed to have been mixed in his tea, his press secretary Kira Yarmash tweeted late Monday.

Why it matters: Navalny is the face of Russia's domestic opposition to President Vladimir Putin and has been jailed dozens of times as a result of his activism. Navalny was hospitalized in July after his doctor said he was poisoned with "undefined chemical substances." Yarmash said he felt ill on a flight from Tomsk to Moscow and the plane made an emergency landing in Omsk, where he was taken to hospital, per a Twitter translation.

Editor's note: This is a developing news story. Please check back for updates.