Critical Mass London: last friday of the month South Bank by National Theatre, 6.30pm

I suspect the Police will be happy to argue that afterwards- right now all they want is to prevent CM crossing the river.

too right, no point arguing with plod unless you want to be arrested. they are not there to argue, that happens after arrest, they will do what they have been instructed to do. and if that is "stop bicycles from crossing the river" then they will do that. Their reason for arrest (rightly or wrongly) will be "suspected breach of a imposed Section 12 order".