Demonstrators briefly invaded the Royal Bank of Scotland, one of Britain's bailed-out banks, where they smashed windows and spray-painted "thieves" on the walls before being pushed away by cops.
After the building was cleared, riot police on horseback arrived to make sure protesters didn't invade the bank a second time.
The building was empty of staff today, "The safety of our staff and customers is of paramount importance and as such, we have taken the precautionary step of closing selected London branches." A RBS spokesman told ABC News.
Other protesters tried surging through the police cordon surrounding the Bank of England, but were repeatedly pushed back. Officers made themselves into a human chain with their arms linked.
Later on in the evening the protesters scuffled more roughly with the police, and set an effigy of a banker on fire. Television pictures showed smoke billowing over the jeering crowd.
A total of 87 people were arrested today, and according to the police 7 were minorly injured. The protesters have dubbed today "Financial Fools' Day."
"The G-20 has attracted an unprecedented amount of interest from protest groups," said Simon O'Brien, a senior commander from London's Metropolitan police.
Thousands of police have been assigned to guard the summit of leaders of the world's wealthiest nations. The police maintain that every eventuality has been considered in this operation that is costing more than $10 million. All manholes have been sealed, snipers have been placed strategically on rooftops and extra forces are on standby.
"This is London's largest, most challenging security operation. In terms of security, I cannot outline our plans, but I can say we have the right amount of officers and expertise," O'Brien said.
All leave for police officers in London has been canceled for today and Thursday and a high-tech command center is monitoring almost every inch of the city with surveillance cameras.
Protesters boast of surprises and countertactics and are coordinating their efforts by text messages, Twitter and social networking sites. This morning a group, caught with police uniforms, drove a riot truck or armored car through the city. The police immediately stopped the truck and arrested those involved; 19 protesters in total have been arrested today.
While so far the protests have been peaceful, police fear that some could turn violent and that this event has brought some anarchists out of retirement.
"There has been a return of familiar faces from the protest scene, faces we haven't seen for years. I am not going to name any, but there are individuals and groups we haven't seen for a while," O'Brien said.
Protests Peaceful but Boisterous
Many of the protesters are wearing black masks. Some are carrying signs that say "Death to Capitalism," "Balls to the Banks," "Abolish Money" and "All You Fascists Are Bound to Lose."
"We have a surge of contingencies to deal with every sort of eventuality. Anyone who is violent will be dealt with in a proportionate manner," O'Brien said.
Four parades, led by so-called Horsemen of the Apocalypse (signifying the death of capitalism), were outside the Bank of England in the heart of London's financial district at noon. In a party-esque atmosphere thousands were gathering there and cheering, "The people united will never be defeated!"
One protester told ABC News, "It's really all about saving the small guys from the bankers who are stealing money from them."
His friend Olly spoke on a more somber note. He was out there to vent his frustrations about the financial system. He used to work at the Tanker Pub in London, but lost his job in December. He has since been forced to move back in with his parents to save money.
"Times are hard," he said, sadly.
Tensions are running high in Britain, one of the countries worst hit by the global financial meltdown. "There are plenty of people who would actually, literally, love to hang bankers. There's no question. I mean, that's the mood at the moment," said Chris Knight from the protest group Government of the Dead.
In separate protests, the group Climate Camp in the City erected scores of tents in the middle of a city street and planned to stay there all night calling for the G-20 leaders to tackle climate change.
Across town The Stop the War Coalition group was meeting outside the U.S. Embassy to march toward Buckingham Palace where President Obama is due to meet the queen later today.
Business as Usual for Banks?
Bankers, lawyers and others who work in the financial district have been told to dress down for work today. Many boarded up their buildings. Some offices even took the precaution of telling staff to stay at home and many advised pregnant staff not to come to work today.
"Bankers have been told that even ripped jeans would be acceptable to wear," one city worker told ABC news. She added that a photo of protesters outside the UBS offices at Liverpool Street in 1999 had been circulated around the office as an example of what to expect.
"It's very quiet so far, but we are apprehensive," said Jeremy Batstone-Carr of the stockbrokers Charles Stanley. "We are not taking anything for granted."
Bankers responded to the 1999 protests by pouring champagne on the demonstrators and throwing fake £50 pound notes out the window. There have been reports today of similar taunts -- although in a perhaps token nod to today's hard times the bankers were apparently waving £10 notes.
The legendary British stiff upper lip attitude was out in evidence. A smartly dressed city worker told ABC News, "I'm perfectly proud of what I am and what I do and what I look like! Why should I be intimidated by people like that?"
"Everyone's turned up to work. It's the old British culture – 'don't let them get to you,'" said David Buik, from the brokerage firm BGC Partners.
Despite the thousands gathered outside, it was "business as usual" today at the Bank of England.
Hoda Farhanghi and Ammu Kannampilly contributed to the reporting of this story.