I wandered through the crowd and saw more stereotypical activists - scary-looking guys in hoodies and camouflage gear waving banners and flags, and lots of dreadlocks on white people.
Most people were there to protest against austerity and human rights issues, but a whole array of other causes were also being supported by small groups around the square. The Hare Krishnas were in the middle, dancing around merrily. The Free Palestine group hung around the back near the National Gallery. The Hunt Saboteurs Group gathered in Northumberland Avenue. Lots of other people wandered around, both masked and bare-faced, protesters and passers-by, many stopping to take photos or video on their iPhones.
The police were there, but kept their presence low-key. From time to time a repeated broadcast would be played, reminding the crowd of the rules - no fireworks, no graffiti, no music, no climbing on monuments. Fireworks were let off from beneath Nelson's Column, but the police knew not to kick up a fuss about minor incidences.
The square was also swarming with reporters and photographers, all eager to get in on the action, or lack thereof.
Some people in the square seemed totally oblivious that anything was going on, while others just sat on benches and watched. One group had brought their dog - he looked as if he'd rather be curled up asleep on the sofa.
Most of the signs people were waving were hand-written and serious; there wasn't much humour on offer. The most common printed sign said "There's One Solution: Revolution." There was the usual bit of banker-bashing here and there ("Save the Animals... Eat Bankers!").
It was a great place for people-watching, and listening to snippets of conversation as I passed people, with very differing sentiments. "It's great to see this leftist unity, bring together all the left-wing causes in one place," I overheard one man say, whilst a minute later another said "these gatherings just bring out all the weirdos and dregs."
After 40 minutes the crowd started heading south, down Whitehall towards Westminster. Behind them they left a trail of empty coffee cups, a few beer bottles and torn posters, but the litter wasn't too bad.
I decided to head home after they started the march, not wanting to get caught up in trouble. I didn't think that there would be much more to see anyway. It turns out that there was a little bit of unrest later on - some barrier rattling and hurling and 10 people were arrested, but otherwise it was a peaceful event. I guess the latte-drinkers didn't really want to get in any trouble, in spite of the anonymity their masks brought them.