The Russian opposition leader Alexei Navalny is plotting to spoil Vladimir Putin’s 65th birthday celebrations with nationwide protest rallies on Saturday, including in the Russian president’s home town.

“Let Putin listen and go into deserved retirement,” Navalny said in an angry message dictated from prison in which he likened the president to a turnip. “He’s been in power for 18 years, which is long enough.”

Navalny, 41, an anti-corruption lawyer who wants to run for president next year, urged his supporters to demand that the Kremlin allow genuine political competition. The opposition leader’s campaign team says protests are being organised in 80 cities across Russia.

The rallies are the latest stage in Navalny’s months-long bid to force his way on to the ballot for the March 2018 presidential elections.

Russia’s government-controlled election committee says he is ineligible to stand for public office because of a previous conviction for fraud. Navalny, who is currently serving his third prison sentence since March, says the charges were trumped up to stifle his political ambitions.

Authorities in St Petersburg, where Putin was born on 7 October 1952, had previously refused to give Navalny permission to meet his supporters on Saturday. “We weren’t intending to offer or provide Navalny with any venue for a rally,” the city’s vice-governor, Konstantin Serov, said. “We will never do this.”

Officials have also refused to sanction protest rallies in Moscow, and police have warned that anyone attending risks arrest. At least 1,750 people were detained in Russia on 12 June during anti-Putin protests called by Navalny.

Putin has not yet officially announced that he will run for a new six-year presidential term in March. Few analysts doubt, however, that the ex-KGB officer will seek to extend his rule to 2024. Putin has called opposition figures such as Navalny “national traitors”.

An opinion poll published by a state-run pollster on Thursday gave Putin an 82% approval rate. About 2% of Russians said they would vote for Navalny. However, Navalny had similarly low ratings before he stood for Moscow mayor in 2013 and he eventually took almost 30% of the vote after running an energetic, western-style election campaign.

Navalny, who is barred from speaking on national television, mocked Putin’s sky-high approval ratings in his message from behind bars.

“This is the same as asking someone who has been fed turnip all their life – how do you rate the edibility of turnip? The rating will be quite high,” he said. “[But] there are better things than turnips … If we don’t do something, they’ll be feeding us that damned turnip all our lives.”

Navalny’s election campaign has attracted tens of thousands of people as volunteers and raised more than £1m in donations from ordinary Russians. It has also provoked violence from pro-Kremlin activists and thugs with suspected links to the government. In April, Navalny suffered serious eye injuries after an assailant threw a chemical into his face.

In September the head of his campaign headquarters in Moscow, Nikolay Lyaskin, was attacked by a man with an iron bar and sustained a concussion. “They are trying to frighten people. They are telling them, if you work against the authorities, you will get beaten on the head with an iron bar,” said Lyaskin.

