Chrism Eucharist at St Paul's

I don't normally see this kind of thing when I pop out of the office for a lunchtime walk, and I'm not entirely sure what is going on. But the sign on the steps says there was a Chrism Eucharist service at 10, so I assume it was something to do with that.

The bishop came out at the end and advised the assembled throng to "return to your constituencies and prepare for power", which seemed to amuse them.