Russian police on Tuesday were searching the apartments of investigative journalists and their relatives as authorities pile more pressure on independent media.

Proyekt (The Project), one of Russia's last remaining independent media outlets specialising in in-depth investigations, said that police were raiding the homes of its chief editor Roman Badanin and journalist Maria Zholobova.

Another journalist, deputy editor Mikhail Rubin, was detained while police were searching his parents' apartment, the media outlet said on social media.

The searches came ahead of the planned publication of an investigation into the alleged wealth of Interior Minister Vladimir Kolokoltsev, his son and other relatives, Proyekt said.

-But we will publish it anyway- the media outlet said on messaging app Telegram.

Kremlin critics say that during his two decades in power President Vladimir Putin has silenced most dissidents and has muzzled the media.

The few opposition and independent media that still operate in Russia are under huge pressure, Kremlin opponents say.

In April, authorities designated Meduza, a popular Russian-language news website based in Latvia, a "foreign agent", forcing it to launch a crowdfunding campaign to survive the loss of advertising revenue.

The next month another independent online media outlet, VTimes, received the same tag and shut down in June.

Groups or individuals identified as "foreign agents" in Russia must disclose their sources of funding and label publications with the relevant tag or face fines.

The designation is seen as a deterrent for advertisers, and staff of publications with the label say the stigma makes it more difficult to work, including quoting sources.

Russia will hold parliamentary elections in September, and ahead of the polls authorities declared the organisations of jailed Kremlin critic Alexei Navalny extremist and barred his allies from running.