Nadezhdin, a former liberal member of Russia's parliament from 1999 to 2003, wrote on Telegram: "The police have come" and that he was being taken to the police station in his hometown of Dolgoprudny, north of Moscow.
He was released a few hours later and ordered to appear in court later this week on charges of displaying "extremist symbols," an administrative offence punishable by a fine or a 15-day jail term.
The charges against Nadezhdin were based on a 2023 online interview in which he briefly showed a picture of the late opposition leader Alexei Navalny, who was then serving a 19-year sentence on charges of extremism widely seen as politically motivated, according to Zona.media, an independent online outlet.
An opponent of the war in Ukraine, Nadezhdin attempted to register as a candidate in Russia's 2024 presidential election but was blocked by the electoral commission, which cited errors among the 105,000 voter signatures he submitted.
The Ministry of Justice accused Nadezhdin of spreading disinformation about decisions made by Russian authorities and of calling for unauthorised demonstrations and vigils.
Last Friday, Nadezhdin was designated a "foreign agent" by Russia's justice ministry, a legal category that has been widely used against critics of the Kremlin.
He has insisted that he would run for parliament in September's election as an independent candidate, even though the "foreign agent" label bars him from running.
The few opposition figures still at liberty in Russia have complained about an increase in the already pervasive repression ahead of the election, which is scheduled for September 18-20.
Despite the widespread dissatisfaction among many Russians about the war, the Kremlin-backed United Russia party intends to maintain its position of power in the election.
Russian political scientist Alexander Kynev said that the crackdown on Nadezhdin was a signal from the government that it intended to deter those with dissenting views from taking part in the Duma election.
with DPA and AP
