Riot police confined most of the activists to Threadneedle Street, next to the Bank of England, who responded with "shame on you" chants.
Police said a number of their officers had been injured in the protests.
In a statement police added there had been "a number of missiles thrown at officers and a number of surges at the police cordons, and increasing levels of violence towards police".
At least two dozen were arrested during the demonstrations, with police mounting one of Britain's biggest security operations.
Demonstrators raised effigies of the "four horsemen of the apocalypse," representing war, climate chaos, financial crimes and homelessness.
Outside the Bank of England an effigy of a banker in a bowler hat hung from a traffic light.
Michael Rainsvro, a protester from the 'G20 Alternative', told Al Jazeera that the demonstrators outside the Bank of England wanted a "real dialogue" about the economic future.
"We are seeing 20 men in a room making decisions for the 20 richest countries in the world, but these decisions effect the entire world," he said.
"We came to the central bank of England to say: this is not just a problem with a few regulators and a few bad bankers, this is a problem that has been going on for years."
Day of action
Shop fronts and restaurants were boarded up to protect them from the protests, and helicopters hovered over the demonstrations.
"I am angry at the hubris of the government, the hubris of the bankers," Jean Noble, one of the protesters, said.
|Activists staged protests against the G20 summit outside the Bank of England [AFP]
"I am here on behalf of the poor, those who are not going to now get their pension or who have lost their houses while these fat cats keep their bonuses, hide their money in tax havens and go and live where nobody can touch them."
A smaller demonstration against Britain's military role in Iraq and Afghanistan attracted several hundred people in Trafalgar Square.
Protesters have planned another day of action on Thursday to coincide with the G20 summit.
The meeting of world leaders is expected to focus on how to deal with the global recession, the worst in 80 years.
Wednesday's protests were minor compared to those during a World Trade Organisation meeting in Seattle in 1999, where about 50,000 people turned out and several hundred people were arrested.