Comedian Russell Brand joined the London arm of the worldwide Million Mask March.
He was pictured alongside protesters and later wrote an opinion piece in The Guardian saying that riots are sparked "when dialogue fails, when they feel unrepresented and bored by the illusion".
The group, which has adopted the image of a smiling Guy Fawkes made famous by the film V for Vendetta, held Bonfire Night protests in central London that saw many of the masked demonstrators clash with police.
Officers tried to move members away from Buckingham Palace as fireworks were thrown at its gates on Pall Mall.
Later, police were filmed moving protesters from a branch of McDonalds. Some angry members of the movement retaliated by trying to regain entry to the fast food chain and were filmed chanting "scum" at the officers.
There were similar scenes in Ireland, Northern Ireland, Madrid and Washington and in cities in Japan, Australia, and New Zealand.
One activist, Sean Roesner, 21, travelled to London from King's Lynn, Norfolk, to attend the demo.
I joined Anonymous because I was arrested under the Computer Misuse Act. I spent eight months on police bail last year and had done nothing wrong. We are here to stand up for what we believe in, to make the world a better place.