Supporters of Anonymous have also protested in Dublin and Belfast as the movement takes part in the global protest against corruption on Bonfire Night.
The MP Pamela Nash has tweeted a picture which from Portcullis House shows the scale of the Anonymous protest in Parliament Square.
The website run by Anonymous for the Million Masks March claims that there will be more than 400 protests around the world.
The Green MP Caroline Lucas has met protesters joining the Anonymous march on parliament.
The comedian Russell Brand has joined Anonymous protesters at a rally in central London.
He tweeted "whatever party they claim to represent in the day, at night they show their true colours and all go to the same party."
Supporters of the online action group Anonymous have taken to the streets of London to show their support for the organisation.
Hundreds of people have gathered in Trafalgar Square and have marched on parliament wearing masks of Guy Fawkes.
The protesters say they are taking part in the Million Mask March which has seen similar demonstrations in America, Australia and Scandinavia.
Anonymous claim the protest is demonstration of support for their organisation and WikiLeaks.
Supporters of the hacking group Anonymous have clashed with police near the Houses of Parliament.
Supporters of the hacking group Anonymous are clashing with police in parliament square.