Almost 50 people were taken to hospital last night after demonstrators clashed with police in the City of London.
Fifteen people were arrested as protesters rampaged through the heart of the British capital, attacking police and buildings with bricks, breeze-blocks and scaffolding poles.
Four police officers and two demonstrators knocked down by police vans were among the 46 needing hospital treatment.
City workers trapped in targeted buildings built makeshift barricades after watching the protesters breaks through police lines.
Upper Thames Street, one of the London's busiest roads, was devastated when around 2,000 protesters taking part in the Carnival Against Capitalism launched a hail of missiles at riot police and windows lining the road.
Billed as an international day of action, the event began peacefully with fewer than 500 people protesting at sites such as the Bank of England in Threadneedle Street and High Street banks on Cheapside.
But by early afternoon thousands had gathered at Liverpool Street railway station and moved off into the City in four sections, intent on targeting what they called "morally and politically-incorrect companies".
Police said all four groups began using "unprovoked gratuitous violence" and were "throwing stuff from building sites - bricks, concrete blocks and other debris".
At one point a number of demonstrators were jumping on the roof of a police van and hurling missiles at other vans driving past.
The first protester was knocked down when one of the groups reached London Wall.
Police said three police vans were withdrawing after being surrounded and attacked when the woman was hit.
"They were reversing slowly out of the crowd and it is thought that a female demonstrator climbed on the back of one of the vans and then fell off and went under the vehicle," a City of London police spokesman said.
"She suffered concussion and we believe a leg injury." Bystander Steve Chop (26) from Brixton, south London, said the woman appeared "badly" injured.
"Two of the vans got away, but one van backed up against the traffic lights and then appeared to go forward, driving over a woman without stopping," he said.
"She was about 25 years old. She was just lying there and appeared to be badly injured. It took about 20 minutes before an ambulance turned up." The second person was knocked down by a Metropolitan Police van which had been called in to assist the City force. This person was also taken to the Royal London Hospital for treatment.
The demonstrators eventually moved on to Trafalgar Square where they continued the protest, surrounded by a heavy police presence.