If Critical Mass starts every last Friday of the month with the gathering under Waterloo Bridge, then last night it started well before 6pm, with small bunches of cyclists ready to celebrate teh 20th anniversary and already making passers by mystified by such meeting.
The anniversary t-shirts came at around that time, and given out for a donation.
At around 7 the mass started to move to the Waterloo roundabout, on Waterloo bridge and towards the tunnel, which this time was closed off and made for a little slow to start, but this allowed everyone a chance to chat and socialise.
The mass rode through the West End and on to Buckingham Palace, where the mass stopped for the usual mini-party, impressing some bystanders along the way – the following link is a youtube video but the comment is just as worth it: http://www.youtube.com/watch?v=Ntzsd_ItljI
It seemed the mass was heading to Knightsbridge but it made a U-turn, heading back to the West End, this time Picadilly, where there was yet another U-turn, and on to Trafalgar Square – another mini party.
From there it head West to Battersea Park, with the mass somewhat lowered in numbers, and from there back to Waterloo.
All throughout the ride, some had their phone sharing location for the whole ride, so late-comers could catch up: 
http://www.glympse.com/!criticalmasslondon how-to
Report on Urban75: http://www.urban75.org/blog/critical-mass-celebrates-its-20th-anniversary-with-a-huge-central-london-ride/
TfL also reported on the ride: twitter TfL
Pictures and maybe video:
http://bambuser.com/v/4566252
Victoria
video: http://youtu.be/L_jWHffIx5E
https://www.youtube.com/watch?v=rY3pvM0lwX0
https://plus.google.com/105941112677430321612/videos
http://connect.garmin.com/group/666216
Comments on Twitter …
Video and comment on YT:
http://www.youtube.com/watch?v=Ntzsd_ItljI
I happened to run into this massive gathering 
of bikers and skaters at Buckingham Palace while I was simply wandering! Turns out it’s the 20th anniversary for a bike ride called Critical Mass. There were COUNTLESS participants. What a wild thing to see!
Some reports and comments from FB:
A great experience overall – thanks everybody
Tonight’s Mass was great!
What a 1 hell of the mass.
Gotta say that was ridiculous fun with some banging tunes to have the mass on another level.
Big thanks to Icame Isawi Steve for making the Mass birthday profile adverts
Loved the party atmosphere despite being slightly damp.
Thought the U turns were chaotically anarchically good humoured awesomeness.
Loved the swarms of people who helped pacify impatient drivers by simply smothering them with quiet passive peaceful numbers.
*most* motorists were quite well behaved, with the occasional exception.
Main thing for next time though; guys at the front, don’t shoot off once you’re through a bottleneck. You need to keep it slow so that no gaps form in the mass, otherwise we just end up spread out and with vehicles able to sneak in.
Thank you London, that was beautiful.
