On the day of the now infamous G20 summit in London I was fully prepared for total chaos in the capital. All types of media – from broadsheet newspapers such as The Times to activist blogs, "Twitter" and everything in between – had been discussing what was due to take place on the streets. It really didn't sound pretty.

April 1, 2009, would see the following events take place, pretty much simultaneously:

• The world leaders that make up the G20 were to drive their bomb-proof, police-escorted, blacked-out cavalcades around the capital's twisting streets to meet up and discuss the state of the world's finances. Crowds and cheering fans would be waiting, cameras at the ready.
• At the same time, multiple demonstrations had been planned by the international anarchist/anti-capitalist community near key landmarks. These ranged from a "camp-in" by the British activist group Climate Camp, to several much more aggressive sounding events, with names like "Eat the Bankers" and even "Class War." Tens of thousands were expected to turn out.
• As if this wasn't enough, that evening the English national football (soccer) team were due to meet the Ukrainian national side at the largest stadium in London, Wembley, for a World Cup qualifier.

For the police, it would be (as one typically British officer put it) "a challenge." Of course, I would be there to see it.

As I approached the financial district on the day, it was clear this was to be no ordinary day of demonstrations. Everywhere on the tube (subway) were signs stating that swathes of the rail network were to be closed "due to police instructions." This meant all underground trains were veritably groaning with irate passengers and by the time I was belched out of the train near the financial district I was glad to come up for air.

I left the train as close as I could get to the main demonstration near the Bank of England and immediately I could feel the tension in the air. No cars lined the streets and windows everywhere were boarded up. Everywhere people seemed to be carrying placards and some people were wearing fancy dress: clown wigs, masks and other clothing eerily out of place in an area usually solely populated with suited bankers. I gradually realized most people were walking away from what seemed to be a faraway sound of a huge crowd chanting and shouting somewhere nearby. Naturally, I headed in the opposite direction to see what was going on.

What greeted me as I approached the aptly named area of "Bank" was a small crowd of disgruntled protesters flanked by a veritable wall of riot vans and police in luminous yellow outfits. On the other side, at the far end of the street behind several layers of hefty police, I could see a very large, very noisy and very angry crowd. There seemed to be lots of red smoke, like at an Italian soccer match. As always in situations like this it seemed best to move on. It was time to find a gap.

I walked counter-clockwise around what appeared to be the bulk of the crowd, negotiating various little side streets all filled with yellow-suited police, some of whom seemed to be heavily kitted out in riot gear to the point that it was hard to tell if they were men or, in fact, large heavily-armed bumblebees. It was not the last surreal sight of the day nor the most extreme.

Presently I came to a street filled with a sizeable crowd. I had entered the financial district proper and all buildings in the area were of the vast, grey, monumental type to be found decorating financial institutions around the world. It was, as I would soon learn, Threadneedle Street, one of the most famous in the history of Britain's banks. It was also about to be the scene of a very large riot.

As I joined the crowds of people it became obvious that I was in fact very close to a serious encounter between police and demonstrators. The street became more and more densely packed and the sound of chanting grew louder. I began to take pictures and soon found myself packed in extremely tightly surrounded by a peculiar mix of young men in masks and hoods, and foreign journalists with expensive cameras. With only a small chink of blue sky visible between the buildings overhead and what felt like a rugby scrum around me, it became more difficult to move.

Slowly I worked my way to the front. And then I can say quite clearly that "it" began. After that things moved fast and it is difficult to put things in precise order.

Roughly speaking, they went like this:

• I became conscious that the group of people in front of me had been trapped (or "kettled") by the police directly outside the Bank of England. Then people started shoving from all directions and shouting. I was conscious of a policeman's helmet being knocked off and police hitting out at everyone with metal batons.
• I became aware that a man around 10 feet or so away from me was covered in blood, seemingly leaking from his head. I smelled acrid smoke and looked up to find the whole wall next to me covered with journalists and people in masks, watching the proceedings while perched on crumbling window ledges.
• A helicopter began to circle overhead, and as I looked up I was conscious of being watched from rooftops several stories up, and also filmed by policemen. Then the group of police in front of me seemed to panic and bundled past me, forcing some other people and myself against a wall.
• I found myself photographing a mob of people as they smashed the windows of the Royal Bank of Scotland – one of Britain's largest banks and the company I entrust with my own savings. Demonstrators entered the building. Furniture was thrown outside. Suddenly, through a broken window I could see police fighting with hooded activists inside the bank and someone was starting a small fire.

Eventually, as I frantically fired off shots with my camera, I was conscious that more and more police were arriving and this time they were on horseback. Was this a dream? Could this really be happening to me today in London? Rather than encounter a large mounted police presence, I pushed my way, rugby-style, through the crowd and kept pushing until I was through the tightest, loudest part of what had now become a mob. Eventually I found myself in a quieter stretch of street and around me things seemed to be calming down. I caught my breath, and tried to make sense of what I had just been seeing.

Then, as suddenly as it had started, the crowds began to part and people started to find a way out. The police seemed relieved and a mini reggae sound system switched on, crackling out a mix of revolution songs, peace and love. The atmosphere that had been intensely violent dissolved and people began to trickle away, heading home.