In London today, Barack Obama and Dmitry Medvedev agreed to make cuts in their nuclear arsenals; Obama and Gordon Brown announced that the G20 were "within a few hours" of agreeing a global plan for economic recovery; and Nicholas Sarkozy, in cahoots with Angela Merkel, threatened to scupper the whole show if his calls for tighter financial regulation are not met. But at 11 am, outside the Bank of England, we waited under an eggshell sky for the Four Horsemen of the Apocalypse: red against war, green against climate chaos, silver against financial crime and black (the website said) against borders and land enclosures, in memory of the Diggers.

War got there first, escorted by a small crowd offering the usual British cocktail of whimsy ("Queers against capitalism and other nasty things" "Eat the bankers") and testosterone ("We are fucking angry"). There were fists in the air, and singing, to the tune of "Clementine": "Build a bonfire, build a bonfire, Put the bankers on the top…" Small knots of anarchists in black drummed up a rapid rhythm; police in day-glo green formed equally rapid cordons; the last red double-deckers tried to nose through the crowd. Everyone was taking pictures, with cameras and mobile phones: if it isn’t mediated, it isn’t happening. "Jump! Jump!" people shouted up at the windowless bank, and "Where’s our money?" and "Shame!"

The protest seemed a broad bricolage of causes: a young man waving a red flag allowed that we’re not in a revolutionary situation yet, "but I think we might be soon"; three feet away, a woman holding one end of a banner ("Capitalism isn’t working") said she was furious with Gordon Brown for saddling her children with debt and may well vote for the Tories in the next election. But Mary–retired, with a "Wage Slave" label on–rebuked my cynicism. "I refute the idea that we’re all talking about different things," she said. "The kind of world we want to see is the same world—a world where money is used to help people. We’re all just talking about different bits of it."

Within the hour all four horses had arrived, and several thousand of us found ourselves penned (or "kettled") by the police in the broad plaza at the end of Threadneedle Street. A woman in business clothes, down for a meeting, had to get home to pick up her baby son from nursery; no dice, the officer said. No one was getting out. Back in February the Met issued dire warnings of an impending "summer of rage". Today they seemed determined to fulfil that prophecy. There’s nothing like being hemmed in to make you want to push back; a panicky anger wells even if you don’t want it to. When the cordon briefly parted, the crowd surged forward, and I saw the first scuffle between a policeman and a protester–no political content there, just two guys losing it.

Once I’d escaped I walked a few blocks to the European Climate Exchange (which trades in carbon credits), where a very different, "fluffier" protest was taking root. A tent city had sprung up between the office buildings, complete with bunting and colourful posters: "Welcome to the Bright Side." Instead of red and black, hot pink and brilliant green; instead of shouting, cake and daffodils–because "Nature doesn’t do bailouts." The police stood around, bemused, with their arms folded.

This was Climate Camp, a network (mostly young and mostly white) of people who’ve gathered regularly since 2006 to talk and to protest–earlier camps were held at Heathrow Airport and at Kingsnorth, where there are plans to build a coal-fired power plant. Dave, a post-doc doing research on carbon dating at Oxford University, sat in the doorway of his tent dressed in a business suit "so the police won’t stop and search me and take all my belongings." This feels to him like the birth of a new movement. "With most protests, people turn up for the day and then go home; this is an ongoing thing. It’s the only thing that makes me feel optimistic, though I don’t know if we can actually stop climate change."

There was a definite buzz here, a purposeful party atmosphere. People talked about reclaiming something that was lost, a sense of ownership of the streets and of the land, about building communities. Young men wandered about offering gingerbread. There were workshops on carbon trading and Copenhagen, Samba and self-defense. Two mermaids in green wigs and long blue sparkly dresses worried about sea levels; a land-based woman wore a T-shirt reading "I heart ethical investment." Apparently there are factions here, as everywhere–"some people want capitalism to end, while others simply want it to take note of science"–but the core ethic is non-violence and consensus building. What made the camp so different from the protests round the corner, which felt, from here, a million miles away? It’s not so simple, they insisted. We’re the same people; it’s just a different style, a different tactic.

Back at the Bank, the police were putting on riot gear; there had been a few things thrown, a few heads cracked. Someone had smashed the window of the Royal Bank of Scotland (whose former director, Sir Fred Goodwin, was rewarded for his failures with a million dollar pension); but as this picture shows, the cameras’ black snouts outnumbered the missiles. The whole thing felt like a painful tempest in a teapot: the simulacrum of a riot, dreamed up by the police and a handful of protesters.

There have been three different demonstrations in London today, in three very different styles: a traditional march to Trafalgar Square let by the Stop the War Coalition, with speeches by the big beasts of the left; the Climate Camp; and the "meltdown" at the Bank. No prizes for guessing which one made the most headlines. On my way home I passed an Evening Standard billboard: "Anarchists battle for City," the big black letters read, as if we were on the verge of civil war.

Update, April 2nd: Yesterday evening the police encircled the Climate Camp and began to crush the entirely peaceful demonstrators gathered there with riot shields and strike them with batons: see this film posted on Indymedia. The campers put their hands in the air and chanted "This is not a riot," but the police continued to shove. They were then kept penned without food or water for five hours before being searched and allowed out one by one. It was also reported this morning that a man died, apparently of a heart attack, during the protest at the Bank of England.