Old Bill pulling up cyclists on City Road this AM

It's an official cut-through for cyclists, it has its own traffic lights. I thought they were bike mechanics at first., then thought they were just offering bike -tagging....then got nervous when I saw they were asking everyone to dismount.

Well they must have had a damn good reason, because surely you're not obliged to co-operate? If Po-po pulled me over and told me to dismount I'd want to know why before I did