Man passes through a London park and notices everyone is staring at him

A Redditor:

So I literally just walked through the garden at the south east corner of Hyde Park - something bloody weird is going down. Having cut across from the north of W. Carriage Drive all was well; saw a few people walking about nothing l noteworthy.

Then having crossed the large path by the lake I come into this garden (towards green park) and some guy walks up and just stares at me, my first though was it was some drunk homeless guy and no big deal. Stare back for a second then walk past him on my way, that when things seriously got creepy. The park was filled, and I mean filled with people doing exactly the same thing. They weren't all dressed like your usual creeps either, guys wearing suits, big black coats these guys (they were all guys) could have been mistaken for a Whitehall staff picnic...all of them stopping and staring at me as I walked through.

I'll confess my pulse increased quite dramatically as they all looked at me, blank expressions, some followed me for a bit others were right at the opposite end standing still or slowly walking about. 

Anyway I stayed on the same path, didn't look back and nothing happend. Two of them tried to speak to me but I had my headphones in and didn't hear

Is some group holding an event there I missed the memo on?! Seriously that's simultaneously the most confusing and creepy thing I have ever seen.

Can you guess what was going on?