Anti-capitalist and anarchist groups have gathered in Canary Wharf for a Halloween party to "dance on capitalism's grave".
"It's another crisis. And now people are actually starting to talk about the alternatives to capitalism," a man who would only call himself Agent Unbuckled said.
"I think the time is right for us. September 11th overshadowed everything. Now is the appropriate time to say 'can we do something to make the world a better place rather than going off and invading countries'."
Businesses around the Docklands area sent warnings to staff urging them to go home early and not wear any clothes bearing corporate logos.
An internal email sent around Lehman Brothers, seen by politics.co.uk, warns staff that demonstrators "may seek to gain entrance to buildings or approach people on the estate in order to conduct trick or treat".
Corporate buildings in the area organised shutdown operations in case demonstrators try to rush the doors.
Patrick Ward of the Socialist Workers said: "Capitalism uses various different means to keep itself going. At the moment it's using massive bailouts from the taxpayer to bail it out and it's going to have to reconfigure the way it operates away from the absolute free market, neo-liberal methods it has been using and towards more Keynesian aspects."
The event was not sanctioned by the Canary Wharf Group, but the Canary Wharf security department is understood to have prepared for the event and liaised closely with the Metropolitan police.
Organisers promised the event would be non-violent, and crowds were cheerful and in a party-going mood as the evening wore on. But there was also a strong police presence.
Nick Rogers of the Communist party of Great Britain told politics.co.uk: "We need to bring the whole left together on a principled basis on which differences can be debated or expressed."
The demonstration, which began when the moon appeared, was organised by an anthropology professor at the University of East London.
There was a funeral march for capitalism, followed by speeches, dancing and a closing ritual.
There was also a two-minute silence for those who have died in Afghanistan.
Lehman Brothers became the epicentre of the financial crisis when it collapsed at the end of the summer.
Many employees have lost their jobs but several are still working in the building.