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I would call them more anti-corruption protests than pro-Navalny (which it was). Either way, the largest number in Russian history after 1991. In 2011 protests were massive, but nowhere near as many arrests. Note that the entire center of Moscow has been blocked (unprecedented on this scale), the metro has been closed, entire streets have been blocked and widespread police violence has been used to reduce the number of protesters. Once they even crammed people into regular commuter buses. Around 80 journalists were arrested, some decent police officers who decided to join the protesters (one waving a gold toilet brush) and random boys, girls, men and women. But the masses are not afraid of the thugs, and the number of arrests is also a good indicator of the crowd – maybe 1 to 100. The regime is clearly afraid of the people and the protests, to go on and grow in size.
You can assume that Russia will soon decouple itself from the global internet, or at least block the main source of communication for protesters, Telegram.