Radiohead's Thom Yorke was joined by Massive Attack's 3D as the pair put on a gig in support of the Occupy London movement.
The secret show, which featured a two-hour DJ set, was held for 100 people in the basement of the Swiss bank UBS's building in East London.
The Occupy movement, who call themselves the Bank of Ideas, took over UBS three weeks ago in protest against government austerity cuts.
Occupy London protester Ronan McNern said the protesters did not court the musicians.
"To be honest, it was actually the artist [who came to us], so Thom from Radiohead and Massive Attack, they wanted to do something for Occupy London. It's not a matter of us organising, it's them wanting to do something.
"What they wanted to do is just say thank you to all the people that have been involved in Occupy London, the Occupy movement, in both the London, the UK and the world," he said.
Another protester, Catherine Garrity, said that the gig featuring world-renowned acts could re-ignite Occupy's momentum.
"Unfortunately people, they don't pay attention to politics. They pay attention to Radiohead. They pay attention to Massive Attack. And I think this will get us out to a different audience."
Radiohead and Massive Attack are the latest well-known acts to join the cause, after playwright Alan Bennett and comedian Russell Brand.
The Occupy London movement are to be given a judgement on eviction from their UBS site today at the Court of Appeal.
One of the most senior figures in City regulation will also meet protesters today from the St Paul's Cathedral camp as part of a Church of England initiative on finance and ethics.