Police in the Russian capital detained at least eight people on June 26 after a Communist-led rally opposing compulsory vaccination against COVID-19.
The gathering attracted several hundred people.
Police reportedly moved in after the event was concluding.
OVD-Info, a group that monitors protests and arrests in Russia, said the eight detainees included the head of the Communist Party faction in the Moscow Duma, Nikolay Zubrilin, and Left Front member Andrey Seleznev.
The demonstration came with COVID-19 death and infection rates in Russia -- and Moscow in particular -- hitting half-year highs.
While Russia mounted a space-race-style effort to bring the Sputnik-V vaccine to authorization, doubts have persisted in many quarters about it and the country's mass inoculation campaign has run up against considerable vaccine hesitancy.
This week, Moscow Mayor Sergei Sobyanin said vaccinations against COVID-19 would be compulsory for 60 percent of employees in the trade and services sectors, education, and health care.
More than a dozen other regions followed suit.
President Vladimir Putin's spokesman, Dmitry Peskov, has said that the vaccination order is not "mandatory" because people who object to the requirement can quit and find other jobs.
Moscow has also imposed stricter limits on gatherings, with inside seating at restaurants and bars as well as some other events closed to those without a QR-code showing proof of vaccination, a recent negative test, or certification that they contracted the virus in the previous six months.
Russia has registered the sixth-highest number of infections in the world with more than 5.3 million cases, and more than 130,000 deaths, although actual figures are thought to be much higher.
Communists Rally Against 'Mandatory' Vaccination In Hard-Hit Moscow
A health-care worker prepares a dose of Sputnik V vaccine against COVID-19. Hundreds of people in Moscow demonstrated on June 26 in opposition to what they said were "mandatory" vaccinations.