Police have warned that violence could break out at an anti-capitalist rally being co-ordinated by Anonymous in central London tonight.
Organisers of the Million Mask March have told attendees "the police are not your friends" as Scotland Yard imposes rigorous restrictions on the protest.
Clad in Guy Fawkes masks, thousands of supporters of Anonymous, the anarchic hacking collective, are expected to attend.
The London protest is one of many similar marches held worldwide on November 5. Its agenda is broadly anti-capitalism and pro-civil liberty.
Last year, 10 people were arrested after vandalism of public buildings and violence towards police officers.
Scotland Yard said the march must not continue after 9pm and must be limited to a section of the West End including Trafalgar Square, where it will start, and Westminster.
Police said past events have seen "high levels of anti-social behaviour, crime and disorder".
"This year we have strong reason to believe that peaceful protest is the last thing on the minds of many of the people who will come along," said Chief Superintendent Pippa Mills.
The Metropolitan Police have briefed businesses along the route of the protest.
Ms Mills added: "It is unacceptable that a small minority should believe they have the right to break the law, harass people, damage buildings and attack police officers.
"My message to those people planning on attending on Thursday night is if you want to take part in peaceful protest, that is your right and we want to work with you. If you want to break the law then we will police you. Where offences are committed, we will investigate them and make arrests."
The restrictions have been imposed through the Public Order Act 1986. Police have reiterated that they are able to remove masks where they think there will be criminal offences.
A Facebook page for the event, run by Million Mask March London, said: "The police are not your friends, this goes for police liaison officers too, they're simply a 'friendly face' who collect intelligence."
More than 20,000 people have said on Facebook that they will attend the event, which is described by its website as the "World's Largest Protest by the Most Influential Group in the World".
Yesterday, a Twitter account which appeared to be affiliated to the Facebook page for Million Mask March tweeted to the Metropolitan Police: "We Are coming in peace ! Please remember this fact ! We Are A Peaceful Collective ! See yal tomora."
The march could mean a second day of trouble for police, following a protest held yesterday by students calling for free education.
Most of the demonstrators were peaceful but a number turned violent as the march passed Government buildings.
Police confirmed they had made 12 arrests for public order offences.