The Home Secretary is addressing the annual conference of the Police Federation in Bournemouth - just days after they took to London's streets to protest against pay cuts and job losses.
Officers jeered as Mr May told them regular fitness tests, new entry requirements to ensure the most talented recruits were hired, and direct entry for experienced individuals into senior ranks should all be welcome changes.
She was also heckled as she said: "It is because the police are crime-fighters that we will never privatise policing."
One officer shouted: "You already are."
Mrs May insisted that she was having to "make difficult decisions on pay" because of "tough" economic times.
She said it was not true that the Government was singling out policing, adding that while the 20% budget cuts were challenging, they must be seen through "for the good of our country".
A Government-commissioned report in March recommended cutting starting salaries, curbing generous pension provisions, clearing the way for compulsory redundancies and penalising unfit officers with pay cuts.
The depth of feeling was made clear last week when more than 30,000 officers marched through central London wearing T-shirts with a picture of Mrs May saying: "Get shafted and carry on."
There were also demands from officers for the right to strike, a form of action they have been banned from taking for nearly 100 years.
Paul McKeever, chairman of the Police Federation, which represents officers up to the rank of inspector, said Mrs May was "on the precipice" of destroying the police service.
He told the conference: "Home Secretary, we warned you about the riots and you said we were scaremongering.
"We warned you that a 20% budget cut would damage the front line and were told we were wrong.
"We are warning you that you are racing towards a train crash that could destroy the effectiveness of policing in this country and harm the safety of the public.
"Home Secretary - take breath. Stop now, and review what you're doing or you will be found guilty of destroying the finest police service in the world."
The police service is facing budget cuts of 20% amid the financial squeeze and police numbers are down to their lowest in a decade at around 136,000, according to the latest police figures.
The federation has protested about what it calls the "privatisation" of police, with the security firms signing multi-million-pound deals to run custody suites and so-called "back office functions".
More to follow...