"Poisoning is becoming a favorite tool of the people who are going after political opponents of Vladimir Putin's regime," said Vladimir Kara-Murza, a Russian opposition politician and vice-president of the Free Russia Foundation, a Washington-based nonprofit.
Navalny, a leading critic of Putin, was fighting for his life in a Siberian hospital on Thursday after drinking tea that allies said they believe was laced with poison.
Kremlin spokesman Dmitry Peskov said any poisoning would need to be confirmed by laboratory tests and that doctors were doing everything they could to help Navalny. He wished him a speedy recovery.
The Kremlin has denied settling scores with its foes by murdering them.
Kara-Murza told Reuters he believes the suspected poisoning may have been connected to Navalny's campaign of support for opposition politicians running in next month's regional elections in Russia, as well as the protests taking place in Belarus over a disputed election.
Kara-Murza, who survived two alleged poisonings in 2015 and 2017, said although poison has been a weapon of choice for the Russian government for many years, the number of incidents has increased dramatically under Putin. He pointed to Yuri Shchekochikhin, a high-profile journalist and opposition politician who died from suspected poisoning in 2003, among others.
The international spotlight "can save a life," Kara-Murza said, adding that democracies in particular must maintain their attention on Navalny. "It's very important to keep the light switched on because it's obviously much easier to commit a crime in darkness."