Russian opposition activist Lyubov Sobol, an ally of Kremlin critic Alexei Navalny, was detained by police on Monday after a weekend protest in Moscow, Navalny's spokeswoman said on Twitter.
A few thousand Russians took to the streets of Moscow on Saturday to demand free elections to the capital's city legislature on Sept. 8, defying a ban enforced by detentions during previous protests.
Protesters have been calling for the release of activists detained in earlier rallies, and Sobol on Saturday described the arrests as "mayhem," blaming it on the city government.
"Lyubov Sobol has also been detained," said Kira Yarmysh, a spokeswoman for Navalny.
Sobol was released a few hours later, she tweeted.
Earlier on Monday evening, Russian journalist Ilya Azar and Nikolai Lyaskin, a Navalny ally, were detained, according to their social media posts.
Azar's wife said that police had left their 2-year-old daughter alone in their apartment with the door unlocked when they detained him. Moscow’s commissioner for children’s rights has asked the city prosecutor's office to investigate the claims.
Lyaskin later tweeted he had been released, and the Apologia Telegram channel said that Azar was also released.
Reuters contributed reporting to this article.