After initially being ejected from the lobby of Millbank by police and security staff, the protestors smashed their way in through windows.
Using sticks and chairs they destroyed CCTV cameras and broke windows from the inside of the office block. Employees had to be evacuated as masked youths rampaged through corridors and onto the roof.
Dozens of police officers stationed outside the entrance were overwhelmed as the crowd of thousands pelted them with rocks and bottles.
BBC video (not embeddable)