People with national flags and various political parties flags gather during a protest in Moscow, Russia, Aug. 10, 2019.
Tens of thousands of people rally in central Moscow, Aug. 10, 2019, for the third consecutive weekend to protest the exclusion of opposition and independent candidates from the Russian capital's city council ballot.

Russia's communications regulator, Roskomnadzor, says it has asked Google to take measures to prevent the advertising of "illegal mass events" on its video-hosting site, YouTube.

Roskomnadzor said on Sunday that it had sent a letter to Google saying that Russia would consider it interference in its sovereign affairs and a hostile influence should the U.S.-based tech giant fail to respond to the request.

The announcement comes a day after tens of thousands of opposition supporters gathered outside Moscow's center for a sanctioned rally demanding fair municipal elections.

Hundreds of people later gathered in more central parts of the city, prompting police to detain more than 250 people, according to the independent watchdog OVD-Info.

Police detain a man during a protest in Moscow, Russia, Aug. 10, 2019.

OVD-Info said 79 people were also detained in St. Petersburg, 13 in Rostov-on-Don, two in Bryansk, and two more in Syktyvkar as "solidarity" rallies attracted smaller crowds there and in other cities.

Earlier on Sunday, Andrei Klimov, head of the Committee for the Defense of State Sovereignty in Russia's upper house of parliament, the Federation Council, said that "foreign opponents took advantage of information and computer technologies in order to manipulate Russian citizens who attended" the unauthorized Moscow protests, TASS news agency reported.

In July, Roskomnadzor fined Google 700,000 rubles ($11,000) for failing to censor content blacklisted by the agency in accordance with strict Russian Internet laws.