Alexei Navalny Health Concerns Prompt Russian Opposition Calls for Protest

MOSCOW—Supporters of jailed activist Alexei Navalny have called for mass protests to demand his release amid growing fears over his health, setting the stage for an escalation in the standoff between Russian President Vladimir Putin and an opposition movement that has struggled to break his hold on power.

Mr. Navalny’s team had said it would call for demonstrations after gathering half a million signatures of people willing to take to the streets. But on Sunday, it reversed course following reports that the activist, who has been on hunger strike for almost three weeks , was in critical condition.

“Events are moving too fast and too bad,” his team said. “It is no longer possible to wait and postpone…his life hangs in the balance.”

On Saturday, a medical trade union with ties to Mr. Navalny cited the results of medical tests they said they obtained from the activist’s lawyer as showing he was at risk of imminent kidney failure, which could lead to cardiac arrest.

The protests, called for April 21 in the central squares of Russian cities, would fall on the same day Mr. Putin is scheduled to deliver his annual address to the Federal Assembly, Russia’s national legislature.