Since you're looking for specific reported numbers, in the UK a rally in Hyde Park attracted between 20,000-30,000 as reported in the Guardian, with the low figure given by the police and the high one by the organisers, which included CND (the Campaign For Nuclear Disarmament) and the Committee to Stop War in the Gulf, an alliance containing elements of the Labour Party, the Socialist Worker Party, and various other groups towards the hard left. This included some sitting MPs, including Tony Benn, Bernie Grant and Jeremy Corbyn.

In terms of size, this is larger than, say the reported 12,000 who marched in support of veganism in a yearly rally in 2019, but is dwarfed by both the 2003 Stop the War protests against the Iraq War and the 2002 Countryside Alliance march protesting a ban on fox hunting

Since you're looking for specific reported numbers, in the UK a rally in Hyde Park attracted between 20,000-30,000 as reported in the Guardian, with the low figure given by the police and the high one by the organisers, which included CND (the Campaign For Nuclear Disarmament) and the Committee to Stop War in the Gulf, an alliance containing elements of the Labour Party, the Socialist Worker Party, and various other groups towards the hard left. This included some sitting MPs, including Tony Benn, Bernie Grant and Jeremy Corbyn.

In terms of size, this is larger than, say the reported 12,000 who marched in support of veganism in a yearly rally in 2019, but is dwarfed by both the 2003 Stop the War protests against the Iraq War (upwards of 750,000) and the 2002 Countryside Alliance march protesting a ban on fox hunting (around 400,000).