If thousands was the best he could get in Belgrade, where the pan-Slavic movement in the EU is at its strongest, Putin isn’t very popular in Europe right now. In comparison, back in 1983, the Russian-funded (on the sly) Campaign for Nuclear Disarmament managed 250K demonstrators in London.
https://en.wikipedia.org/wiki/Campaign_for_Nuclear_Disarmament#The_Second_Wave:_1980%E2%80%931983
The Serbs are traditional allies of Russia. However, the Russians were unable or unwilling to stop the NATO intervention in ex-Yugoslavia in the 1990s. Also. Ukraine is also Slavic and Orthodox Christian, not Catholic or Germanic.