Members of activist group Anonymous - notable for their signature Guy Fawkes masks - have been urged to co-operate with police officers over a planned demonstration tonight which could cause gridlock in central London.
Scotland Yard has also warned the group it has powers to remove face coverings ahead of the so-called 'Million Mask March', which could see thousands of protesters descend on Parliament Square.
A statement on the Anonymous website, which claims responsibility for the march, said the group will have:
In a statement, the Metropolitan Police said they had attempted to contact organisers of the event "without success".
And it added that they have imposed Section 60AA of the Criminal Justice and Public Order Act 1994 across Westminster between 5pm today and 2am tomorrow, which provides powers to remove masks when police fear a crime will be committed.
Anonymous says it is protesting against austerity, infringement of rights and mass surveillance.
Last year's Million Mask March saw a handful of protesters charged after hundreds descended on Buckingham Palace and Parliament Square.
Officers made 15 arrests during last year's protest, which saw demonstrators clashing with police and a fire started near the palace, as well as damage to Nelson's Column and the Victoria Memorial.