Two close associates of Alexei Navalny have been detained ahead of protests planned to support the imprisoned Russian opposition leader, who has been on a hunger strike since March 31.

Mr Navalny's team had called on Sunday for nationwide protests after reports about his health deteriorating in prison. Russian authorities have stressed that the demonstrations were not authorised and warned against participating in them.

Vladimir Voronin, a lawyer for top Navalny ally Lyubov Sobol, said on Twitter that people "in uniform" removed Ms Sobol from a taxi near a Moscow metro station on Wednesday morning and took her to a police precinct.

Police also detained Navalny spokeswoman Kira Yarmysh, who is currently under house arrest on charges related to January protests in support of the politician.

Ms Yarmysh was detained near the entrance of her apartment building when she went out during the one hour she is allowed to leave, her lawyer, Veronika Polyakova, said on Twitter.

Mr Navalny was arrested in January upon his return from Germany, where he had spent five months recovering from a nerve agent poisoning he blames on the Kremlin. Russian officials have rejected the accusation.

His arrest for an alleged violation of a suspended sentence from a 2014 embezzlement conviction triggered protests that represented the biggest show of defiance in Russia in recent years.

Soon after, a court ordered Mr Navalny to serve two-and-a-half years in prison for the embezzlement conviction, which the European Court of Human Rights deemed to be "arbitrary and manifestly unreasonable".

Mr Navalny began the hunger strike to protest against prison officials' refusal to let his doctors visit when he began experiencing severe back pain and a loss of feeling in his legs. The penitentiary service has said Mr Navalny was getting all the medical help he needs.

Mr Navalny's physician, Dr Yaroslav Ashikhmin, said on Saturday that test results he received from Mr Navalny's family showed sharply elevated levels of potassium, which can bring on cardiac arrest, and heightened creatinine levels that indicate impaired kidneys.

"Our patient could die at any moment," he said in a Facebook post.

On Sunday, the politician was transferred to a hospital in another prison and given a glucose drip. Prison officials rebuffed attempts by his doctors to visit him there.

In response to the news about Mr Navalny's health, his team called for more nationwide protests on Wednesday, the same day that Russian President Vladimir Putin is scheduled to deliver his annual state of the nation address.

According to a website dedicated to the protests, demonstrations were being planned in 165 Russian cities.

Meanwhile, the Moscow prosecutor's office asked a court to brand Mr Navalny's Foundation for Fighting Corruption and his network of regional offices as extremist organisations.

Human rights activists said such a move would paralyse the activities of the groups and expose their members and donors to prison sentences of up to 10 years.