Up to 16,000 off-duty officers will don black caps representing each officer expected to be lost under the Government's budget cuts as they take to the streets today.
The officers, banned from striking under law, will march through central London in a protest against proposed changes to their pay and conditions.
Some 20,000 officers from all 43 forces across England and Wales are expected to take part in the first police march in the capital for more than four years, organisers said.
They include 400 from Hampshire, 200 from Dorset, 650 from Thames Valley Police, 700 from Sussex and 470 from Kent.
The Police Federation, which represents rank-and-file officers, said the sea of black caps will show the number of officers the public will lose over four years as a result of the cuts.
Paul McKeever, the federation's chairman, said: "The march is the only way that police officers can demonstrate their anger.
"The reality of the cuts to policing is really beginning to bite; numbers are beginning to fall rapidly.
"In the past year alone we have lost over 5,200 police officers from the frontline and we are witnessing the privatisation of core policing roles as chief officers struggle to cope with budget restraints.
"The Government need to be realistic about the outcome of severe cuts to policing; we cannot afford to compromise on public safety."
Once 16,000 officers wearing black caps are on the streets, others will don white caps instead, organisers said.
Home Secretary Theresa May, who asked former rail regulator Tom Winsor to carry out the most wide-ranging review of police pay and conditions in 30 years, will address the officers at their annual conference in Bournemouth next week.
The route of the march, from Millbank to Waterloo Place in central London, will see the officers pass both the Home Office and Downing Street. It is billed as a protest to show "the unprecedented attack on policing by this Government and the consequences that these cuts will have for public safety".
The last time police took to the streets, then-home secretary Jacqui Smith was blamed for a high-profile pay dispute in January 2008 and was ridiculed at the federation's conference.
A Home Office spokesman said: "As a service spending some £14 billion a year it is right for the police to make their contribution to reducing the record budget deficit.
"Existing police pay and conditions were designed more than 30 years ago which is why we asked Tom Winsor to carry out his independent review.
"We will continue to ensure that police officers are rewarded for doing an exceptional job."