More than 600 antiwar protesters were detained in Russia hours after President Vladimir Putin ordered Russia's first military draft since World War II, according to Russian news outlet SOTA Vision.

Most of the detainees are in Moscow and St. Petersburg, according to the news outlet.

Sergei Shoigu, Russia's Defense Minister, on Wednesday said the country would draft 300,000 reservists to support its military campaign in Ukraine, hours after Putin said the additional manpower was needed to win a war against not only Ukraine but also Western backers of Ukraine.

Shoigu said Russia had 25 million potential fighters at its disposal.

The decree published on the Kremlin's website said the call-up would apply only to reservists with previous military experience.

Shoigu said this meant around 300,000 men. He said they would be given additional training before being deployed to Ukraine, and that they would not include students or those who were currently serving as conscripts.

"These are really those who have served, have a military specialty, that is, a specialty that is needed today in the Armed Forces, who have combat experience. Those who are serving under conscription — it doesn't apply to them either," Shoigu said. "They are not subject to any mobilization and direction for the 'special military operation.' Our conscripts also continue to serve, as they did, on the territory of the Russian Federation."

Video footage by SOTA Vision showed protesters clashing with police officers at an "anti-mobilization" rally in St. Petersburg.

Unsanctioned rallies are illegal under Russia's laws.

Russia's opposition on Wednesday called for protests against Putin.

"It is clear that the criminal war is getting worse, deepening, and Putin is trying to involve as many people as possible in this," Kremlin foe Alexei Navalny said in a video message from jail recorded and published by his lawyers.

"He wants to smear hundreds of thousands of people in this blood," Navalny said.

Information from Reuters was used in this report.

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