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.
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.