I just gathered it from live video, local forum posts, news and my intuition of the size of Hong Kong island (it's all full with people still being stuck in the train system from Kowloon side). From images, it's also visually much bigger than the previous ~500K turnout protest.

But yeah it's intuition after all so I could be wrong. Still, numbers in the 60K range is just impossible.

Edit: By gather and intuition, I mean I'm getting quite a volume of confirmation bias from forum post, video comments and the hosts of the protest.