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Anti-capitalist protesters embarked upon a wrecking spree within a City branch of the Royal Bank of Scotland today, and engaged in running battles with police as G20 demonstrations turned violent.

Police were forced to use dogs, horses and truncheons to control a crowd of up to 5,000 people who marched on the Bank of England, in Threadneedle Street, on the eve of the London summit.

As officers on the steps of the Bank held firm, several hundred demonstrators broke away to move towards the RBS building on the corner of Bartholomew Lane.

Officers attempted to control them but a hardcore group of 20 still got into the building, which had been closed for the day.

The rioters then went on the rampage within the bank, which was at the centre of a row over the £700,000-per-year pension of Sir Fred Goodwin, its former chief executive.

Telephone lines were ripped out, office furniture wrecked, windows smashed and graffiti daubed on both the inside and outside. A blue office chair was used to smash up one of the blacked-out branch windows, while another chair was thrown out of the window.

Police officers eventually managed to drag the protesters out after 15 minutes and drive the crowd away from the building shortly afterwards. All of those involved in the incident were believed to have been arrested.

RBS said it had closed the branch in advance over security fears, and added that employees were not at work when the violence broke out. "We had already taken the precautionary step of closing selected City of London branches, including the Threadneedle branch," a spokesman said.

Meanwhile, the protesters who marched to the Bank of England were still being penned in this evening by officers who cordoned off surrounding streets, amid what Scotland Yard called "increasing levels of violence". At least one officer was injured.

The demonstrators – a combination of anarchists, anti-war protesters, environmentalists and others – had made their way to the bank using four different routes, from Moorgate, Cannon Street, Liverpool Street and London Bridge.

A Met Police spokesman said: "There have been a number of missiles thrown at officers and a number of surges at the police cordons, and increasing levels of violence towards police. Cordons have been put in place to maintain safety and control the crowd."

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