Russian opposition leader Alexei Navalny, a staunch Kremlin critic, has been detained in advance of a rally in a major city.

Navalny on Friday posted a video on his Instagram account of what he said were officers outside his home in the capital, Moscow, asking him to come to a police station. He tweeted from the station later, saying he had not been told why he had been detained.

Navalny had planned to travel to the city of Nizhny Novgorod, Russia's fifth-biggest city, where he was to lead a rally later in the day.

The rally had been due to start at 6pm (15:00 GMT) but Moscow police said Navalny was detained "over multiple calls to participate in an unauthorised public event".

The head of Navalny's campaign, Leonid Volkov, was detained in Nizhny Novgorod.

The Tass news agency on Friday quoted police as saying that Navalny was detained because of his calls for unsanctioned rallies, which is punishable by up to 30 days in jail.

Navalny, however, insisted the event was authorised.

"This is, of course, a new level. Now they are detaining me for an attempt to take part in a sanctioned meeting," Navalny said on Twitter.

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

Previous detentions

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 to stand for office" because he is currently serving a five-year suspended sentence for embezzlement.

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.

OPINION: Youth vs Putin - 2:0

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.