It was more than 300k, I’m no expert but my reference was the Anti-war protests that opposed the Iraq intervention, and whilst it wasn’t bigger than that, it was close.

Also it was totally peaceful, no police around. Police presence increased near the end, on Vauxhall bridge road, but just to block people from spilling into some sidestreets (not all).

Police were friendly to be fair. Looks like all the trouble they had was with Cruella sympathisers. Go figure.