MOSCOW (Reuters) - Jailed Kremlin critic Alexei Navalny said on Friday he wanted it known that he had no plans to commit suicide in prison, as he issued a message of support to his followers on the eve of protests the authorities say are illegal. Navalny was detained on Sunday after flying home for the first time since being poisoned with what the West says was a military-grade nerve agent that Navalny says was applied to his underpants by state security agents. The 44-year-old lawyer, in a Moscow prison pending the outcome of four legal matters he describes as trumped up, accuses President Vladimir Putin of ordering his attempted murder. Putin has dismissed that, alleging Navalny is part of a U.S.-backed dirty tricks campaign to discredit him. Navalny’s allies plan nationwide protests on Saturday to try to force the Kremlin to order his release. The authorities have opened a criminal case into protest organisers, accused Navalny’s allies of trying to illegally encourage minors to attend, and warned that attendees risk catching COVID-19. Navalny, in a message on Instagram via his lawyer, said he wanted people to know he was in good physical and mental health. “Just in case, I am announcing that I don’t plan to either hang myself on a window grill or cut my veins or throat open with a sharpened spoon,” the post said. “I use the staircase very carefully. They measure my blood pressure every day and it’s like a cosmonaut’s so a sudden heart attack is ruled out. I know for a fact that there are many good people outside my prison and that help will come,” he wrote. The authorities have made it clear they will crack down on Saturday’s protests. On Friday, Moscow courts sentenced Navalny’s spokeswoman to nine days in jail, gave another ally 10 days, and fined another 250,000 roubles for what they said were illegal calls to attend the protests. Navalny supporters outside Moscow were also targeted and Russia said that TikTok and other social media had deleted what it called illegal posts promoting the protests. Maria Zakharova, a foreign ministry spokeswoman, said on Facebook: “When minors are being invited to (political) meetings do not expect anything good. Because children are dragged into political battles when all other options are not working, which means that the ideologues of this process are ready to do anything to achieve their goals.”