MOSCOW (Reuters) - Russian news outlet Open Media said on Wednesday that its website had been blocked for most users by Russia's communications watchdog after it entered the site in a list of internet pages engaging in "extremist activity".

       Writing on its Telegram channel, the news outlet, which was founded by Kremlin critic Mikhail Khodorkovsky, said it had not been informed by the watchdog of its decision to ban the site.

       Instead, the outlet said it had received several letters on Wednesday asking it to remove content from its site that was against the law, without indication of the content in question.

       Numerous investigative media organisations have been outlawed in Russia in recent months as part of a widening crackdown ahead of September's parliamentary election that has targeted media regarded by authorities as hostile and foreign-backed.

       Russian authorities last month labelled some journalists from the Open Media outlet, as well as from several other non-state outlets, as "foreign agents".

       The term carries negative, Soviet-era connotations and subjects those designated to extra government scrutiny and labelling requirements.

       The MBKh media outlet, also founded by Khodorkovsky, said it had been blocked by the watchdog too.

       "At the moment it cannot be opened using most (internet) providers in Russia," the outlet said in a post.

       Media affiliated to Open Russia were limited following a request from the General Prosecutor's Office, the RBC news outlet cited Russia's communications watchdog Roskomnadzor as saying on Wednesday.

       (Reporting by Maria Tsvetkova; Writing by Polina Ivanova; Editing by Marguerita Choy)