The home secretary is "on the precipice of destroying a police service that is admired and replicated throughout the world", rank-and-file officers have claimed.
Theresa May's reforms, some of the most radical changes in more than 30 years, will fundamentally change the dynamics of policing, the Police Federation said.
The warning comes as Mrs May prepares to address the federation's annual conference in Bournemouth, less than a week after 30,000 officers marched through London to protest against changes to pay, terms and conditions.
The last time police took to the streets, then-home secretary Jacqui Smith was ridiculed at the same conference as she was blamed for a high-profile pay dispute in January 2008.
Paul McKeever, the federation's chairman, told the conference: "This is a bad deal for the police service.
"We have less resilience, fewer warranted officers, a weakened front line and a radically altered model of British policing.
"You are on the precipice of destroying a police service that is admired and replicated throughout the world."
He added: "Home secretary, we are seeing proposals, things being put in place without infrastructure, that will fundamentally change the dynamics of policing.
"You cannot expect officers, those who understood the fiscal situation and accepted that some cuts were necessary, to take an unfair share of the cuts and just sit there and be content with their lot."
Shadow home secretary Yvette Cooper said the government was cutting too far and too fast and officers across the country knew their forces were "facing a cliff edge, worried that the service to the public is falling".
Her comments echoed those of Gloucestershire Chief Constable Tony Melville who will step down at the end of the month in opposition to the election of police and crime commissioners.
In February, Mr Melville warned that further budget cuts put the force on a "metaphorical cliff edge" and was in a "perfect storm".
Ms Cooper also called for the government to do more, not less, to help officers injured in the line of duty.
"When a police officer, seriously injured in the line of duty, is determined to return to the policing job they love, they should not be penalised," she said.
"i think they deserve the confidence of knowing their force will back them all the way."
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."
A podium has been hastily arranged for Mrs May after aides asked for her to be allowed to stand throughout her speech and Mr McKeever's, rather than sitting down as she and other home secretaries have done in previous years, sources said.
But she will still stand on the stage, where the backdrop says "Cutting police by 20% is criminal".
Mrs May was greeted by officers standing and holding banners saying "Enough is enough" and "Cutting police by 20% is criminal" as she walked on to the stage.
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