Alexei Navalny, the leader of Russia's opposition Progress Party and founder of the Anti-Corruption Foundation, posted on Twitter that police detained him in the entrance of his apartment block in Moscow to interview him at a police station.

"I was detained at the entrance. They are driving me somewhere 'for explanations,'" said Navalny, who linked to a video on his Instagram account of police detaining him.

He was leaving to get a train to a political rally in the provincial city of Nizhny Novgorod that was set to take place later Friday.

Read more: Kremlin clamps down on opposition leader Alexei Navalny supporters

The opposition leader later wrote that police had not given him a reason for the detention. Russia's TASS news agency cited the press service of the Russian Interior Ministry as saying Navalny had been detained because of his "repeated calls to take part in unsanctioned public events."

Russian opposition leader Alexei Navalny was to attend a rally before being detained

"The Kremlin views my meetings with voters as a huge threat and even an insult," Navalny wrote in a post on Twitter. "They were saying for so long that opposition has no support in the regions, and it now pains them to even look at our rallies."

Authorities in Nizhny Novgorod said they had refused permission to hold the event. Earlier on Friday, Navalny posted on Twitter said police had "stolen" equipment for the rally. His supporters have vowed to go ahead with the rally, which they said was approved by city hall.

Opposition protests must be pre-approved, authorities said, though Navalny has previously said that the Russian constitution allows for such events to take place. He has been arrested twice before and served sentences of 15 days and 25 days for organizing unauthorized protests.

A Russian presidential election is slated for next year and incumbent President Vladimir Putin is widely expected to run. Navalny, a staunch critic of Putin, hopes to challenge the Russian president despite the country's election commission having deemed him ineligible to take part because of a suspended prison sentence, one that Navalny says was "politically motivated."

dv/sms (AFP, Reuters)