Alexei Navalny's private doctors have urged him to call off his hunger strike as soon as possible after the jailed Kremlin critic began refusing food three weeks ago.

Cardiologist Yaroslav Ashikhmin and four more doctors said in a statement that they are asking President Vladimir Putin's most prominent domestic opponent "to immediately halt the hunger strike to preserve your life and health".

"If the hunger strike continues even a little longer, we will simply have no-one to treat soon," the physicians said in the statement published by Mediazona, an independent Russian news website.

On 31 March, the 44-year-old opposition politician went on hunger strike to demand proper medical treatment for back pain and numbness in his legs and hands.

Navalny's doctors said thanks to the "huge" public support he was taken to a civilian hospital in the central city of Vladimir and received access to "something that looks like an independent examination".

The doctors added that they had received the results of Navalny's health assessment and his medical tests on Thursday and would inform the opposition figure of their opinion the following day.

Last weekend, Navalny's doctors said he could die "any minute", pointing to the opposition politician's high potassium levels.

On Wednesday evening, thousands of Russians took to the streets across the country, after the West warned the Kremlin that it would face "consequences" in the event of Navalny's death.

More than 1,900 people were detained at Wednesday's protests including more than 800 people in Saint Petersburg, according to OVD-Info which monitors opposition protests.

In a new post on Instagram on Thursday, Navalny said he was full of "pride and hope".

Navalny's right-hand man Leonid Volkov said that the opposition politician's doctors had received information about his health because the Russian authorities responded to public pressure.

"It is very sad that one has to refuse food for 23 days to achieve such a result," Volkov said in a post on messaging app Telegram.