Masked anarchists smashed their way into the Royal Bank of Scotland branch in the City of London.
They trashed equipment, daubed graffiti on the walls and threw a chair through a window.
More than 4,000 demonstrators, most of them peaceful, gathered near the Bank of England to demand action from world leaders.
But several hundred clashed with police, who were pelted with eggs, paint bombs and empty beer cans.
Scotland Yard responded by cordoning off streets and sending in riot police and officers on horseback.
Police said a total of 24 arrests were made today.
Two people were held for aggravated burglary over the raid on the RBS building and two for public order offences.
Others were held for threatening behaviour, possession of drugs, violent disorder and blocking a road.
Eleven people were arrested this morning after police uniforms were found in the back of an armoured vehicle.
The day began with marches from Cannon Street, Liverpool Street, Moorgate and London Bridge, led by effigies of the Four Horsemen of the Apocalypse.
As the lines of protesters walked peacefully towards the Bank of England, City workers were seen waving £10 notes at them from office windows above.
Political protesters carrying banners marked "Balls to the Banks" and "Punish the looters" were joined by workers who had lost their jobs in the recession.
Former financial worker Caroline Carter, 49, from Chertsey in Surrey, said: "Believe you me, this is going to get a lot worse by the end of this year.
"I suggest the bankers give up some of their pensions and give something back to the community."
Christine Hickey, 64, from Sutton, Surrey, said: "I'm just fed up with the bankers who've got away with taking all this money.
"People are losing jobs and their homes and I'm fed up with it.
"I'm not here with anyone today but wanted to come along to show my support. It all seems good-natured."
When the four processions converged on the roads around the Bank at around midday, the atmosphere turned menacing.
Police closed off nearby streets to stop anyone leaving the area as small groups of protesters began charging police lines.
To cheers from parts of the crowd, a handful of men with black scarves obscuring their faces and hoods over their heads smashed a hole in the windows with a metal pole and crawled in.
Police entered the branch at around 2pm and shortly afterwards began driving demonstrators away from the building.
An RBS spokeswoman said the branch was closed today and there was no-one inside.
"We made a decision yesterday to close various branches, so that branch is closed, there is no question about that," she said.
The outbreaks of violence marred otherwise peaceful G20 protests around London.
In nearby Bishopsgate, hundreds of environmental protesters blocked the road with a climate camp.
They pitched tents and set up market stalls, bunting and banners outside the European Climate Exchange.
One protester, Richard Howlett, said efforts to restart the world economy risked making climate change worse.
He said: "Anybody with any common sense can see that we're not going to get ourselves on the low-carbon economy we need if we're building new runways and new coal-fired power stations."
Campaign group Stop The War said there were 5,000 protesters in Trafalgar Square for an anti-war rally.
Convenor Lindsey German said: "The message is very simple - we want troops out of Iraq and Afghanistan, an end to the siege of Gaza, stop the arming of Israel, and the creation of jobs, not bombs."
At the Bank of England a street party continued, with music blaring.
A short while earlier police moved to once again block all the roads around the Bank, allowing protesters out but not allowing anybody else in.
Groups of demonstrators, passers-by and spectators gathered at the police lines.
Stuart Fraser, of the City of London Corporation, said: "The police are managing a very difficult balancing act: they need to make sure the demonstrators can protest peacefully and safely, whilst protecting property and keeping the City open for business.
"Obviously the operation is ongoing and the situation may change, but so far the police seem to be handling a tricky job very well.
"The City of London Corporation, which acts as the local authority for the Square Mile, is working closely with the police to support their efforts to keep the City working.
"The City of London Police is working in partnership with the Metropolitan Police and British Transport Police in a single operation. By pooling resources and intelligence, we are aiming for an effective pan-London policing operation."
Metropolitan Police Commander Simon O'Brien warned that some of those involved in violent disorder are being monitored by police and face arrest as they move around the city this evening.
Speaking at New Scotland Yard tonight, the senior officer said post incident investigations are already under way against those involved in the attack on a branch of the Royal Bank of Scotland (RBS), criminal damage and assaults on police.
He said small pockets of criminals were responsible for the outbreaks of trouble while the vast majority of participants were good humoured.
Mr O'Brien, who is a member of the command team for the G20 security operation, said some of the apparent ringleaders were followed by helicopter as they left flashpoints.
He said: "As we went on it was clear there were people within the group that were first of all involved in juvenile and puerile behaviour.
"That started to escalate into quite provocative behaviour towards police lines. There were small groups charging forwards and backwards into police lines.
"It did seem to us from CCTV and police on the scene that they tried to find a way to ramp up the protest and hijack it into violence."
Mr O'Brien added: "There are a number of times when police lines withdrew for two, three or five feet to give space between police and demonstrators to take the temperature out of the crowd dynamic.
"Consistently there were people in the crowd who were determined to hijack it.
"We believe these were the sort of people we have been monitoring on the internet and online prior to today.
"Some of these seemed to be quite experienced and determined protesters in that they wanted to get involved in a violent protest and not lawful protest."
Mr O'Brien said police have collected footage from "helitelly", the force's nickname from CCTV from powerful helicopter cameras.
He said: "We started to see a good deal more violence coming from small groups in the crowd. Clear attacks on police line, clear attacks on police by protesters and completely unprovoked.
"Then we saw a determined attack on the RBS where at least three plate-glass windows were broken.
"There was a clear attempt to throw lighted material in that premises and we saw small amounts of minor thefts from the premises.
"That went on for some time and we were in a position where we had officers at and around the scene of that attack.
"We had almost all of it on helitelly and CCTV and we were collecting clear and good evidence of that attack.
"Two people were arrested for aggravated burglary because they entered with a view to steal or cause other forms of damage."
Mr O'Brien said there had been 26 arrests overall linked to the protests, including four last night, and others involved in violence should expect a "knock on the door".
He said: "We are already in the process of collecting key evidence against individuals involved in violent acts and disorder and other activity in the crowd.
"We will pursue a post-incident investigation and any number of people will be visited by the Metropolitan Police and colleagues in the City in the near future.
"If they have been involved in such acts we will arrest them."
Senior officers said one police officer is in hospital tonight receiving treatment after receiving a blow to the head. Seven protesters have also been taken to hospital.
Mr O'Brien warned there may be more violence tomorrow as the focus of the protests moves to the Summit itself at the Excel Centre in London's docklands.
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