Moscow (AFP) - Russian opposition leader Alexei Navalny, who aims to unseat Vladimir Putin in presidential elections next year, was detained by police ahead of a rally on Friday and could face up to a month in jail.

In the latest attempt to thwart the Kremlin critic's campaign, police held Navalny after he left his apartment with the aim of attending a meeting in the provincial city of Nizhny Novgorod.

Moscow police said Navalny was detained "over multiple calls to participate in an unauthorised public event."

Navalny was released in the evening, telling journalists waiting for him outside the police station, "I feel fine."

"We are not going to stop what we do, whatever the obstacles," he said, adding that he would to fly to Orenburg in southwestern Siberia on Saturday and to the northern city of Archangel on Sunday.

The 41-year-old stands accused of repeatedly violating a law on organising public meetings -- punishable by up to 30 days in jail. His spokeswoman, Kira Yarmysh, said he had been instructed to appear before a judge at 1200 GMT on Monday.

Navalny was detained prior to arriving at his last two rallies in Moscow on March 26 and June 12, serving sentences of 15 days and 25 days for organising unauthorised protests.

Navalny said earlier he did not receive any official explanation for being held.

"I am sitting in a reception room and looking at a portrait of Putin," he said on Twitter.

The anti-corruption crusader linked his detention with another -- bigger -- rally scheduled in Saint Petersburg, Russia's second city and Putin's hometown, on October 7, the president's birthday.

Navalny's campaign chief, Leonid Volkov, was detained in Nizhny Novgorod ahead of the rally, saying on Twitter he would be held overnight before a court hearing Saturday.

The city authorities said they had refused Navalny permission to hold the event. Some 200 to 300 people turned up, an AFP photographer at the scene saw.

- Underdog -

Navalny has said he wants to stand for president next March, but the authorities have said he is ineligible because he is serving a suspended sentence for fraud.

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Putin, who has led Russia since 1999, is widely expected to seek and win another six-year Kremlin term.

Despite his underdog status, Navalny has travelled across Russia, drawing crowds of supporters.

Many applaud Navalny -- whose ally Boris Nemtsov was assassinated in 2015 --- for keeping up the fight when many other dissenters have chosen to leave the country or stay quiet.

Political observers say that verbal threats from Kremlin supporters against Putin's critics is giving way to physical attacks.

After one such assault, Navalny was left almost blind in one eye and had to go to Spain for surgery.

The human rights commissioner at the Council of Europe, Nils Muiznieks, urged Russia on Friday to revise its laws on public events, saying it violated the country's international obligations and its own Constitution.

The council's decision-making body, the Committee of Ministers, last week urged Russia to allow Navalny to stand for election despite his "arbitrary and unfair" conviction for fraud.