Police in Russia arrested several opposition leaders at a rally against President Vladimir Putin in Moscow on Saturday.
Alexei Navalny said on Twitter he had been detained at the rally in Lubyanka Square. Ilya Yashin, Ksenia Sobchak and Left Front head Sergei Udaltsov were also arrested.
The BBC reports the rally was one of several that marked a year of protests against Mr Putin.
Mr Navalny later tweeted that he and the other opposition leaders had been released without charge.
Police said a total of 40 people were arrested.
The rally took place outside the headquarters of the Federal Security Service, which is the successor to the Soviet KGB agency.
The Interfax news agency reported other rallies were held on Saturday in St Petersburg, Tomsk and Yekaterinburg.
Earlier this week, Mr Putin suggested that some opposition politicians were being paid by foreign interests.
The BBC reports legal action against opposition figures has increased since he was re-elected to a third term in March.
A new law has been passed on public order offences and curbs have been placed on non-governmental organisations.