The battle of Threadneedle Street will not be remembered in history books but it was certainly ugly.

Four good-natured marches against war, capitalism, homelessness and climate change had converged on the Bank of England. Four thousand people were exercising their right to protest. Then, at noon, a small group of hooded youths, bent on confrontation, surged towards a police line stretched across Threadneedle Street between the high-walled Bank and the Royal Exchange. The demonstrators allegedly threw a canister of CS gas. A policeman was injured. Helmets were ripped from officers’ heads.

The police responded by cordoning off the entire square in front of the Bank and preventing any of the protesters from leaving.

Shortly before 2pm windows of a Royal Bank of Scotland building were smashed — Sir Fred Goodwin’s RBS being seen as a prime example of capitalist greed. Soon bottles, cans and paint bombs were raining down on the police. Protesters chanted: “Our streets” and fought with police. Several retreated with blood streaming from head wounds.

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Baton-wielding riot squads and police on horseback arrived. They slowly pushed the baying mob back down Threadneedle Street while helicopters hovered and police dogs drove intruders from the RBS building. The crowd, including many innocent students, pensioners and legitimate critics of capitalism, were kept penned in the square without food and water for many hours.

The police brought in Portaloos after some had used the Bank of England’s walls. “For years they’ve pissed on me. Now I get to return the favour,” said a young man called Dave, from Bristol, as he zipped up his flies. Others chalked graffiti on those walls: “People will stop robbing banks when banks stop robbing people.”

As evening fell, the police finally began letting people leave, but sporadic skirmishes continued.

One man collapsed and died, apparently from natural causes, yards from a riot police cordon. The man, believed to be in his 30s, was spotted by a member of the public, who told the police shortly before 7.30pm. The Directorate of Professional Standards at the Metropolitan Police Service and the City of London Police have been informed. The Independent Police Complaints Commission has also been informed and an inquiry will be held.

Scotland Yard said that eight people, including a police officer, were taken to hospital, and thirty-two arrested, but predicted more arrests when troublemakers were identified from police film. “It will be for us to decide when we knock on their door and say hello,” Commander Simon O’Brien, of the Metropolitan Police, said. Another spokesman said that the cordons were used “due to high levels of violence against officers in isolated incidents”.

The political damage was much greater as the scenes of violence were broadcast around the world. Those scenes were hardly typical. The only other arrests in London were of 11 self-styled “Space Hijackers”, who drove a legal secondhand armoured vehicle, complete with gun turret, into the City. They were detained for wearing fake police uniforms.

Around the corner from Threadneedle Street a Sixties-style “Climate Camp” was pitched. They brought bunting, picnics and even a small turf lawn. Several thousand antiwar protesters marched peacefully from the US Embassy in Mayfair to a rally in Trafalgar Square.

The City had prepared for trouble. Some banks and shops had boarded up their windows. Many City workers stayed at home; others “dressed down”, although they were still conspicuous in designer jeans and Barbour jackets.

But until the eruption in Threadneedle Street the protests had been remarkably good-natured. United by a ragbag collection of grievances, they carried banners that ranged from the extreme (“Abolish money”) to the faintly humorous (“Capitalist bwankers” and “Swindlers’ list”).

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