Labour leader Ed Miliband is urging the government to rethink plans to outsource more police services to private companies after the G4S Olympic security "fiasco".
In a speech he called the discovery that too few staff were in place to guard venues a "scandal".
And he called for a halt to all new policing contracts until there had been a review of G4S's "ability to deliver".
The government accused Mr Miliband of "cynical opportunism" and "posturing".
Policing minister Nick Herbert said outsourcing had been introduced by Labour and cancelling contracts "would mean sworn police officers doing behind the scenes jobs" and "fewer police officers out on the frontline".
Last week, it emerged that the armed forces were on standby to provide an extra 3,500 troops to help with security at the London Games, amid fears that private contractor G4S would not be able to provide enough trained staff in time.
In April, about 550 civilian staff at Lincolnshire Police transferred to G4S as part of a £200m contract to provide services - including human resources and IT - over 10 years.
Addressing a gathering of his party's candidates to be the first elected police and crime commissioners (PCCs) in England and Wales, Mr Miliband said the G4S story "raises wider questions about the outsourcing of policing services all over the country".
"Public-private partnerships are an important part of modern policing, as the last Labour government recognised. Where there are clear savings - such as police IT - PCCs [Police and Crime Commissioners] should have that flexibility," he said.
But he said "core policing functions", such as beat patrol and criminal investigations, should be "protected from privatisation".
And he urged the government to rethink its approach to outsourcing and called for private firms supplying policing services to be directly accountable to the Independent Police Complaints Commission.
Most outsourcing of police services is in areas such as cleaning, catering and other "back-room" work. Some forces, though, have used companies for custody services and prisoner escort.
Mr Miliband, who said G4S events "beggars belief", argued that the coalition's police budget cuts of 20% in England and Wales by 2015 were resulting in forces "coming under pressure to outsource on a scale and at a speed never before seen".
"Not only that, we are not getting the reassurance we need that core policing functions will not be privatised - there is a complete lack of oversight from the Home Office," he added.
'Get a grip'
He said that he was not opposed to private sector involvement, but said it should be restricted to back office functions, such as providing computer systems.
Mr Miliband also said the government had been presenting private sector involvement as a "magic solution" to police funding cuts but "no-one sees it as a magic solution any more".
Policing minister Nick Herbert said G4S's failures were "lamentable" and they would be held to account - but it was a "one-off event" which did not mean other contractors had failed.
"There is no doubt failure here but to draw wider conclusions is merely opportunistic," he told the BBC News channel.
It was standard practice for private firms to supply security guards to major sporting events, said the minister, adding: "Was anybody seriously saying these guard duties, with thousands of personnel, should have been done by police officers?"
And he stressed that private security personnel would never be used in frontline policing roles or to carry out duties normally performed by warranted officers and would be restricted solely to "back office" functions.
"With crime falling, Ed Miliband should have been congratulating the police, instead we got cheap opportunism," he added.
The row came as a report by the Commons Public Accounts Committee criticised the "last-minute scramble to make sure there will be enough security personnel on the ground" at the Olympics.
Its chairman, Labour MP Margaret Hodge, said the chaos over G4S' security contract was "predictable and undermines confidence in those responsible for managing the Games".
She urged the Home Office to "get a grip" on the situation.