Two persons were arrested in central Moscow on Wednesday for "an attempt to carry out unauthorized acts," a spokesman for the city police told Interfax.
The spokesman said neither of the detainees was the opposition leader Alexei Navalny.
The arrests followed Navalny's application for registration as a candidate in the Moscow mayoral race. Navalny was arrested when he was leaving the Moscow Election Committee building after filing his application, but was soon released.
There were reports that, after Navalny's release, police detained a member of his election campaign committee.
A police vehicle drove up to a group of activists, including Navalny, who were walking through the city center and handing out pro-Navalny leaflets, and Nikolai Lyaskin, a Navalny campaign committee activist and member of the governing body of the Solidarity opposition party, was detained.
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