Russian Opposition Leader Navalny in Coma With Suspected Poisoning

Russian opposition politician Alexei Navalny takes part in a rally to mark the 5th anniversary of opposition politician Boris Nemtsov's murder and to protest against proposed amendments to the constitution, in Moscow, Russia, on Feb. 29, 2020. Shamil Zhumatov/Reuters

MOSCOW—Russian opposition leader Alexei Navalny—widely regarded as a prominent critic of Russian President Vladimir Putin—was admitted to hospital in Siberia on Thursday suffering symptoms of what his spokeswoman called poisoning after his aircraft made an emergency landing.

Navalny, 44, began feeling ill on a plane to Moscow on Thursday morning after drinking tea at an airport cafe in the Siberian city of Tomsk.