Russian police officers detain protesters during a march against the alleged impunity of law enforcement agencies in central Moscow, following the arrest on dubious charges of journalist Ivan Golunov, June 12, 2019. Getty

Moscow -- A peaceful march in support of Ivan Golunov, a prominent Russian investigative journalist who was controversially arrested last week on drug charges and beaten in custody, has ended with mass arrests by Russian police. The march went ahead in spite of the fact that Golunov was unexpectedly released on Tuesday.

At least 423 people were detained at the march, according to civil rights group OVD-Info. Most of them were charged with participating in an unauthorized rally, a misdemeanour punishable by a fine or administrative arrest. Prominent Russian opposition leader Alexei Navalny and a dozen journalists from both Russian and foreign media outlets were among those detained.

Golunov was arrested last Thursday. The police claimed to have found drugs on him and in his apartment. He insisted the contraband was planted on him and the arrest was merely retaliation for his work. Golunov is well-known for his extensive reporting on corruption in the local Moscow government. His latest investigation was into connections between high-ranking government and law enforcement officials and Russia's shady funeral industry.

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His arrest sparked a huge, unusually united outcry from the normally-fragmented Russian media community. Starting the day after Golunov was detained, journalists and activists rallied in front of the police headquarters day and night, demanding his release.

Russian investigative journalist Ivan Golunov, charged with attempted drug-dealing, stands inside a defendants' cage during a hearing at a court in Moscow on June 8, 2019. Getty

Dozens of media outlets released statements in support of Golunov, and the three biggest independent newspapers came out on Monday with the same front page, declaring in unison: "We are Ivan Golunov."

The march on Wednesday was initially planned to mount pressure on the authorities to release Golunov, but in an unexpected turn the police announced they were dropping the charges against him on Tuesday, just hours before the demonstration was to take place.

"Clearly, he was released… in order to (prevent) our gathering," journalist and TV anchor Mikhail Fishman told CBS News on Wednesday.

Golunov was released Tuesday night, and some of his supporters called for the Wednesday rally to be cancelled. But the organizers decided to go through with it anyway.

"We're absolutely happy that Ivan is free, but our goals haven't been achieved yet," Elizaveta Nesterova, one of the journalists who helped organize the demonstration, told CBS News. "We will demand an investigation into the fabricated criminal case, and all such cases in Russia."

Police said 1,200 people participated in the march, but activists and media reports put the number between 2,500 and 3,000. The protesters had no posters or banners; they just strolled down the streets, occasionally chanting slogans as they were monitored by a dozens of police.

The police seemed to go after people at random. In most of the situations that CBS News observed, officers targeted young men walking quietly with the rest of the crowd or even standing apart from it.

While it was widely covered by independent and foreign news organizations, the protest got little to no mention on Russia's state-run media outlets. The majority of them focused instead on the festivities taking place in Moscow to mark Russia Day, a national holiday celebrated all over the country on Wednesday.