More than 20,000 off-duty police officers took to the streets as they warned that government cuts are putting public safety at risk.

Many of the officers, from all 43 forces across England and Wales, donned black baseball caps with the words "Cuts are criminal" as they marched through central London to protest against the spending cuts and wide-ranging changes to their pay and pensions.

Police Federation chairman Paul McKeever told them:

"We care very deeply about the communities that we serve. We have seen what happens when we have a government that has given policing a very low priority.

"If you are cutting our jobs, then you are cutting the service we can deliver and the public's safety is at risk."

The officers, banned from striking under law, began marching from Millbank at around noon in 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.

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.

Policing Minister Nick Herbert wrote an open letter to all officers telling them he and the Home Secretary were "constantly impressed by the work you do for your communities" but insisting that "all organisations have to keep pace with the modern world".

The government wants to recognise the professionalism of officers, reduce bureaucracy and ensure officers are rewarded "for the excellent job you do", he wrote.

But difficult decisions on pay and pensions are needed and, as the service spends some £14bn a year, it would not be right if officers were exempt from this, he added.

He insisted that officers will continue to earn more than other emergency services, continue to take overtime and continue to retire earlier than most people in the public sector.

Police pensions will also still be among the best available, he wrote.

Herbert sought to reassure officers over the greater involvement of private firms in policing, saying: "It will continue to be a public service, accountable to the people."

Private firms can provide some services to create savings, he wrote, but "private contractors are not, and will not be, permitted to exercise the powers of arrest and detention given to sworn officers, beyond the limited detention and escort functions already allowed".

He went on: "Policing is, and will remain, a public service, and the office of constable will remain the bedrock.

"We must take some tough decisions and do the right thing for the whole country.

"But I want to assure officers that we will continue to value, in the Prime Minister's words, the finest police service in the world."

Prime Minister David Cameron's official spokesman said:

"The government inherited a very tough fiscal challenge. We are having to make spending cuts across the board.

"We think the reductions in spending on the police are challenging but manageable and that the police will still have the resources that they need to do the important work that they do."