Navalny's right-hand man Leonid Volkov says a massive rally is planned on Wednesday evening "not just about Kremlin critic's freedom, but his life" after his doctors said he could die any minute as his health deteriorates rapidly in prison.
The team of jailed Kremlin critic Alexey Navalny called for massive protests across Russia next week to help save the life of the hunger-striking opposition leader, who doctors warn could die in a matter of days.
Navalny is President Vladimir Putin's most prominent opponent and the street protests are to be held just hours after the Kremlin chief is set to deliver his long-awaited state of nation address on Wednesday.
"It's to time to act. We are talking not just about Navalny's freedom but his life," Navalny's right-hand man Leonid Volkov said on Facebook.
Volkov said Wednesday's rally, set for 7PM local time, could become a decisive battle against "absolute evil" or Russia's last opposition rally for years to come.
"Call on all your acquaintances and take to central squares," wrote Volkov, who heads Navalny's regional offices, adding that the protests should be massive.
"Don't think that nothing depends on us."
Authorities have ramped up pressure on Navalny's supporters in recent months, detaining more than 10,000 protesters at opposition demonstrations in January and February.
US warns of 'consequences'
The 44-year-old Russian began a hunger strike on March 31 to demand proper medical treatment for back pain and numbness in his legs and hands.
On Saturday, Navalny's doctors said his health had rapidly deteriorated and demanded prison officials grant them immediate access.
"Our patient can die any minute," cardiologist Yaroslav Ashikhmin said, pointing to Navalny's high potassium levels and saying he should be moved to intensive care.
Both France and Germany on Sunday joined a growing international chorus of protest at Navalny's treatment.
US National Security Advisor Jake Sullivan warned on Sunday that Russia will face "consequences" if Navalny dies.
"In terms of the specific measures we would undertake, we are looking at a variety of different costs that we would impose, and I'm not going to telegraph that publicly at this point," Sullivan told CNN.
"But we have communicated that there will be consequences if Mr. Navalny dies."
Sullivan said the White House had told the Kremlin that Moscow would be "held accountable by the international community" for what happens to Navalny while in Russian custody.
EU to discuss Navalny situation on Monday
EU foreign ministers will discuss the case of Navalny when they hold talks on Monday, Germany said.
"At tomorrow's Foreign Affairs Council in Brussels, EU foreign ministers will also address Navalny's situation," German Foreign Minister Heiko Maas told Bild newspaper on Sunday.
Earlier, his French counterpart Jean-Yves Le Drian also said the bloc was watching Navalny's situation closely, as he warned of possible further sanctions against Russia.
"We have already taken action," the French minister said, referring to EU sanctions.
"The package of sanctions is already significant, but there may be others," warned Le Drian.
Maas also demanded that Russian authorities give Navalny swift access to proper medical care.
"We urgently demand that Alexey Navalny receive adequate medical treatment and access to doctors he trusts," Maas told Bild.
"His right to medical care must be granted without delay."
Navalny, Putin's most prominent opponent, was arrested in January on returning to Russia from Berlin after recovering from a near-fatal poisoning attack he says was orchestrated by the Kremlin.
Moscow denies the allegation.
He is serving two-and-a-half years on old embezzlement charges, which he says are politically motivated, in a penal colony in the town of Pokrov around 100 kilometres (60 miles) east of Moscow.