It was reported on R4's Today programme a few minutes after the arrests that the police may have found items which could be used by protesters to chain themselves to lampposts and other immovable street furniture or joining themselves together in a human chain across the route.
They appear to be things that were wrapped around the placards. They could have been chains etc which were just holding the placards together so they were easy to carry before handing out. Or they might have been for another purpose.
One side is mistaken or lying. Who knows, which it is?
People have the right to peaceful protest - and peaceful doesn't mean standing there like a dummy. And the police have gone in there with their hobnail boots on many occasions going back years. Often their arrests have resulted in few, or sometimes no charges, but the protest is ended anyway. But they also have to police it which includes preventing an obstruction.
I don't know.