Former US vice president Al Gore on Friday said he understood the Occupy Wall Street movement's anger at some of the "insane" elements of US corporate and political culture.

The Nobel Prize winner, Apple board member and environmental activist provoked guffaws at a digital technology conference in Hong Kong with a withering attack on the worst side of US capitalism.

"What were they thinking?" he said of the bankers involved in the sub-prime mortgage bubble that burst in 2008, triggering a financial downturn that is still playing out in the form of economic turmoil around the world.

He said investigations showed that many of the traders and bankers whose decisions led to the disaster knew that what they were doing was "toxic", but they didn't care as long as they made a "quick buck".

They exchanged emails with acronyms such as LQTM, standing for "laughed quietly to myself", and IBGYBG, or "I'll be gone you'll be gone", in reference to the risks they were taking, he said.

Gore has previously thrown his support behind Occupy Wall Street, writing a blog earlier this month calling it a first step toward fixing a democracy in crisis.

He repeated the "broken democracy" argument in Hong Kong on Friday, but said capitalism remained the "best system".

"If we are going to be committed to capitalism, as I am, we have to make it sustainable," he said.

Gore served as vice president from 1993 to 2001 under Bill Clinton, then remained in the public spotlight through his environmental activism, including a starring role in the Oscar-winning documentary "An Inconvenient Truth."

He shared the 2007 Nobel Peace Prize with the UN Intergovernmental Panel on Climate Change.