Russian opposition leader Alexei Navalny was released by police on Thursday less than an hour after he was detained in Moscow.

Navalny tweeted that he had been released and given a document on legal procedures against him for organizing unsanctioned protests.

"I don't know what it was and why seven people detained me," Navalny tweeted.

The anti-corruption campaigner has been repeatedly jailed for organizing unauthorized protest rallies throughout the country. He aimed to run for president in next month's election, but was denied a place on the ballot because of a fraud conviction that many view as politically motivated.