Riot police have been sent in after G20 demonstrators attacked a uniformed officer and stormed a branch of the Royal Bank of Scotland in central London.
Protesters smashed windows at the RBS building and clambered inside.
Sky News crime correspondent Martin Brunt said he had seen police CCTV pictures of the demonstrators removing equipment and starting a small fire.
Police on foot backed up by a line of mounted officers lined up outside the branch as smoke bombs were thrown by a baying crowd.
Earlier, a protester hit an officer with a large pole during large-scale protests ahead of the G20 summit of world leaders.
Anger at bankers' role in the global economic downturn and frustration at slow progress in tackling climate change prompted thousands to take to the streets in the capital.
Four protest marches converged on the Bank of England after setting off along separate routes as helicopters circled overhead.
A group of officers was forced to retreat behind metal crowd barriers outside the Bank, apparently because of the crush of the crowd in front of them.
Missiles - including fruit - were thrown towards police as red smoke rose above the crowd.
One protester, who was bleeding from a wound to his head, was seen shouting at police officers who had formed a line in front of them.
Another demonstrator dressed in black then ran forward and hit an officer with a pole.
Sky News Online's Alex Watts, at the scene, said police cordoned off the area where the attack took place.
He said some 50 officers ran in and made arrests.
Scuffles also broke out between police and demonstrators near the Corn Exchange.
Sky's Ian Woods spoke to people who said they wanted to go home but were not being allowed to leave the area.
Earlier, City workers leaned out of windows and waved £10 notes at demonstrators on the streets below, who responded with jeers and shouts.
Protesters set up tents outside the European Climate Exchange for a camp aimed at demonstrating their dissatisfaction with the global approach to tackling climate change.
One of them told Sky News he and his friend had travelled from opposite ends of the country, and hoped to use "peaceful resistance" to stay there for 24 hours.
Another protester, unemployed Adam Lambert, 25, of Finsbury Park, north London, said: "I'm here with the Stop the War Coalition because we think the G20 are not representing the ordinary people in the world.
"We think they are representing the rich.
"Every day we hear of billions being given to bankers and billions are being spent on wars.
"We want to demonstrate today to say we are not going to put up with this and the G20 should represent us.
"I think people are angry and they want to show their anger."
Many shops and businesses closed for the day, while those that remained open braced for violence.
Many City traders dressed down to avoid confrontation with demonstrators, swapping their suits for casual clothes.
Some were told to stay at home, others that they should bring lunch with them to the office.
Simon Denham, head of City finance firm Capital Spreads, told Online: "(It was) an extraordinary trip into work today - not a suit in sight as just about everyone who works in the City, or at least those who haven't stayed at home, came in wearing just jeans and a shirt."