Ivan Golunov’s arrest on drug-dealing charges is seen as an attempt to silence his reporting

Thousands of protesters are to march in Moscow in support of Ivan Golunov, an investigative journalist arrested on controversial drug-dealing charges that are widely seen as an attempt to silence his reports on corruption.

More than 20,000 people have so far expressed an interest on Facebook in attending the march on Wednesday, which has not been sanctioned by the authorities. The protest will pass by the headquarters of the FSB state security service, before ending outside the interior ministry, which oversees the work of the police. Protests were also expected to take place in a number of other towns and cities across Russia, as calls grew for the charges against Golunov to be dropped.

Unauthorised protest rallies are often broken up by the police in Russia. Golunov, who faces up to 20 years behind bars if found guilty, said officers planted drugs on him. He is currently under house arrest.

Facebook Twitter Pinterest Valentina Matviyenko, nominally the third most powerful politician in Russia, said: ‘This is a very bad story.’ Photograph: Anton Novoderezhkin

Valentina Matviyenko, the speaker of the upper house, on Tuesday became the most influential figure so far to defend the journalist.

“This a very bad story,” said Matviyenko, a longtime ally of Vladimir Putin. “There are many questions that the public must receive clear and intelligible answers to. This is either unprofessionalism, recklessness or a provocation.”

Matviyenko, who is nominally the third most powerful politician in Russia, said the case had been taken under the personal control of Yury Chaika, the prosecutor general.

The Kremlin admitted on Monday that mistakes may have been made in the handling of the case.

Golunov, who works for the Latvia-based Meduza news website, was beaten by police and kept in custody for 12 hours, said his lawyer, Sergei Badamshin, after being detained in central Moscow on 6 June.

The police’s case against Golunov appears to be particularly flimsy. Badamshin said forensic tests had not detected the journalist’s fingerprints on the bags of drugs purportedly seized from his home. Previous tests failed to reveal traces of drugs in either the journalist’s urine or on his fingers.

Shortly after his arrest, police published photographs that they said showed a drug lab at his rented flat. However, the photographs were swiftly deleted from the interior ministry’s website after a police spokesman admitted they had been taken at a different location and bore no relation to the charges against the journalist. Police described the incident as “a small mix-up.” Human rights groups said police in Russia often plant drugs on suspects.

Facebook Twitter Pinterest A woman holds a poster reading ‘Take away your drugs, return freedom to Golunov’ during a protest in Moscow. Photograph: Yuri Kochetkov/EPA

Golunov’s treatment has been condemned by figures across a wide spectrum of Russian society, including Kremlin-loyal media. Dozens of journalists from state news agencies have signed an open letter calling for the charges against him to be dismissed. Presenters on Channel One, state media’s flagship channel, have also demanded a transparent investigation into the case.

Dmitry Peskov, the Kremlin spokesman, denied media reports that the government was seeking to “settle” the Golunov case before Putin’s annual question-and-answer show with the public on 20 June.

On Monday, the Russian investigative media outlet Project had cited unnamed Kremlin officials as saying the presidential administration wanted police to drop charges against Golunov because they were worried Putin would face awkward questions about the journalist during the live call-in show.