MOSCOW - Russian opposition leader Alexei Navalny, who aims to unseat Vladimir Putin in presidential elections next year, was detained ahead of a rally on Friday, raising the possibility of a month in jail.

In what is the latest attempt to thwart the 41-year-old Kremlin critic's campaign, Navalny was held in his building's entrance hall as he was leaving to get a train to Nizhny Novgorod, a provincial city.

The rally had been due to start at 6 pm (1500 GMT) but Moscow police said Navalny was detained "over multiple calls to participate in an unauthorised public event."

Officers accused him of repeatedly violating a law on organising public meetings, which is punishable by up to 30 days in jail. The head of Navalny's campaign, Leonid Volkov, was detained in Nizhny Novgorod.

The opposition leader said he had not received any explanation for why he had been held for several hours. "I am sitting in a reception room and looking at a portrait of Putin," Navalny said on Twitter earlier.

He urged his supporters to assemble anyway and also linked his detention with another - bigger - rally scheduled in Russia's second city of Saint Petersburg, Putin's hometown, on October 7, the Russian strongman's birthday.

Navalny, an anti-corruption crusader, has said he wants to stand for president next March, but the electoral authorities have said he is not eligible because he is serving a suspended sentence for fraud.

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

After Navalny declared his bid he was hit by a wave of legal obstacles and attacks and even had to travel to Spain for eye surgery after one assault left him almost blind in one eye.

Navalny has been briefly imprisoned before. He was detained prior to arriving at his last two rallies in Moscow on March 26 and June 12, both of which were not authorised by the city.

He served sentences of 15 days and 25 days for organising unauthorised protests.

Nizhny Novgorod authorities said they had refused Navalny permission to hold the rally but his supporters have vowed to go ahead anyway.

"The Kremlin sees my meetings with the electorate as a huge threat and even an insult - after all no one goes to their rallies without being paid," Navalny said.

"They've said for a long time that the opposition has no support in the regions and now it hurts them to see our rallies."

He called on his supporters to show up at the rally in Nizhny Novgorod.

"Come for the sake of principle and as a sign of protest against the stupidity, senility and degradation that have overtaken our country," he wrote.

Undeterred by the seemingly predetermined outcome, Navalny has pressed ahead with his presidential bid and travelled around the country.

Navalny has been gathering crowds of supporters across Russia, seeking to shift public attitudes and battle political ennui in places such as the Pacific port of Vladivostok and other cities.

Last week the Council of Europe's decision-making body, the Committee of Ministers, urged the Russian authorities to allow Navalny to stand for election despite his suspended sentence, saying he and his co-defendant, former business partner Pyotr Ofitserov, continue "to suffer the consequences of their arbitrary and unfair convictions."

The Russian justice ministry accused the Council of Europe of putting political pressure on Moscow ahead of the elections.

Political observers say that the growing atmosphere of intolerance towards dissenters has prompted a surge in radical feeling in Russia as verbal threats from Kremlin supporters give way to physical attacks.

Many have applauded Navalny - whose ally Boris Nemtsov was assassinated in 2015 - for his decision to keep up the fight at a time when many have chosen to leave the country or stay quiet.