Kremlin critic Alexei Navalny appealed to Russians from behind bars on Thursday nearly a year after he was poisoned with what the West says was a nerve agent and told them to vote tactically in elections next month to try to hurt the Kremlin.

His smart voting plan is one of the last levers Navalny and his allies have after a crackdown this summer outlawed his movement as "extremist".

His allies are banned from taking part in the Sept. 17-19 election, and United Russia, which supports President Vladimir Putin, is expected to win despite a slump in its popularity.

The election is seen as a dry-run for presidential elections in 2024. Putin, who has been in power for more than 20 years, has yet to say whether he plans to run again.

"They've declared half the country extremists to grab all the constituencies...," Navalny, wrote in a post on Instagram.

"They haven't let the strong candidates (run) in the election.... they're scared of smart voting," said Navalny, who has published online posts via his lawyers since being jailed in February for 2.5 years for parole violations he calls trumped up.

Friday will mark the first anniversary of his poisoning, something he blames on the Kremlin. It dismisses what happened to him as a Western-backed smear campaign to damage Russia.

His voting campaign requires followers to sign up and be allocated a candidate who is judged to have the best chance of defeating United Russia in their area.

Navalny's allies say the campaign has come under government pressure.

Police this week came to the homes of at least 300 Navalny supporters listed in a database of registered supporters that was leaked in the spring, according to the OVD-Info protest monitor.

The authorities say Navalny and his allies are extremists intent on destabilising Russia.

Leonid Volkov, a Navalny ally, told Reuters he thought the authorities might block the website used to organise the smart voting campaign.

"We're already seeing loads of (measures by the authorities) and the degree of hysteria is only going to grow in the coming month," he said.

In 2019, Navalny declared his smart voting tactic a success at local Moscow elections after 20 candidates backed by his plan won seats in the city legislature.

A Kremlin source played down the idea of the plan as a threat. The source said the Kremlin was more concerned by discontent over stagnant or falling living standards.

"Smart voting is not such a big problem for us in terms of the country. Moscow, St Petersburg - yes, there might be problems here, but not for the other regions," the source said.

"The problem that worries the presidential administration more is ... disgruntled people. That could influence the results. But I think United Russia will probably still keep a majority."

United Russia secured a constitutional majority in the last parliamentary elections in 2016, but its rating stood at 27% earlier this month, its lowest in 13 years, according to a state pollster.