Comments from feedback form - "There must have been restricte..."
The 20k to 100k figures are conflicting reports as-to the number in-attendance.
For countries with more authoritarian backgrounds (like Russia), the police in representing the state, will downplay protest - giving the 20k figure. The upper estimate, 100k, generally comes from organisers who wish to bolster the import of their cause. If I recall correctly, in this case, the BBC correspondent estimated numbers around 50k, which is probably closest to reality.
There can be contrary situations where an authoritarian regime may up their estimate of protesters to justify heavy-handed tactics, or cover up police failure to prevent actions like looting.
Numbers at rallies, protests, and riots, are often an interesting thing to look at regarding who claims what count.