Russian opposition activist Alexei Navalny was briefly detained in Moscow on Thursday for organizing anti-government protests, he said in a series of tweets.
Navalny's detention comes as campaigning gets underway for Russia's March 18 presidential vote, which the activist is banned from contesting.
Navalny, the best-known political opponent of President Vladimir Putin, has been forced off the ballot because he carries a fraud conviction. He and activists have dismissed the conviction as a groundless and politically motivated move.
He has been arrested many times, including in January, when he led anti-Kremlin rallies calling for a boycott of the vote.
The activist said on Twitter that he was detained by seven people as he came out of a dentist's office after receiving treatment for a toothache.
He was told that legal proceedings had begun against him over the protests, he tweeted.
"I didn't get what it was all about and why 7 people were there to detain me," he wrote.
In 2014, Putin signed into law tough restrictions on protests and public gatherings.
Navalny has risen to prominence in recent years by posting investigative stories online about the alleged corrupt practices of Russia's elite.
Authorities recently shut down his website as well as several social media accounts.
The Kremlin rejects allegations of widespread high-level corruption and condemns Navalny as a dangerous threat to the country's stability.