Ivan Safronov was arrested last week on suspicion of leaking secrets to a NATO country, but the Kremlin says the allegations do not relate to his work as a journalist.
More than a dozen former colleagues of the ex-journalist Ivan Safronov have been detained by police while protesting against his arrest.
Monday's protest outside the Lefortovo pre-trial detention centre took place nearly a week after the reporter was taken into custody outside his Moscow home, suspected of treason.
Many of the demonstrators work for Kommersant, the business daily where Safronov was previously employed.
Some of his former journalism colleagues claimed the authorities may have wanted to take revenge for Safronov's reporting that exposed Russian military incidents and opaque arms trade deals.
"The biggest problem is that we still don't know the basis on which the charges were brought against him and the fact that we may never know this," said Sofia Samohina, a journalist with the Kommersant newspaper who took part in Monday's rally, "because the charges are very serious so the process will continue to be held behind closed doors."
Another Kommersant colleague, Alexander Chernykh, was wearing a T-shirt with the slogan "Freedom to Safronov" as he was led away by officers.
Both the FSB and the Kremlin have said the information Safronov allegedly provided contained important military and security details and did not relate to his work as a journalist.
But his lawyer Oleg Eliseev told Euronews last week that the time frames identified in the allegations coincide with when he was working for two major daily newspapers.
It appears likely that the details of the case will be classified under secrecy laws.
Safronov made his name through his reporting on military issues before becoming an adviser to the Roscosmos space agency.
He claimed in a Kommersant article published last year that Russia had signed a deal with Egypt to deliver Su-35 fight jets.
The article was subsequently removed from the newspaper’s website but reportedly prompted an investigation by the FSB.
Safronov was dismissed from Kommersant later in the year after writing an article that incorrectly predicted the Russian upper house speaker was about to leave her post.
He subsequently became an adviser to the head of the Roscosmos space agency, which said the treason charges did not relate to his work for it.