Delegates from up to 64 private security companies were attending a "bidders' conference" on Tuesday for a £1.5bn contract to run a wide range of policing services in the West Midlands and Surrey.

The contract notice drawn up for the groundbreaking contract invites bids from the private sector for services that include investigating crimes, detaining suspects and managing high-risk offenders.

The widely drawn West Midlands/Surrey contract notice says that all those services that can "be legally delegated to the private sector" have been put on the table while "preserving the integrity of the office of constable".

The West Midlands police authority says there are "many household names" among the 64 firms but has declined to name any of them. Potential bidders will hear both chief constables and the outgoing chairmen of the West Midlands and Surrey police authorities detail the procurement process and outline what they hope to achieve from the "business partnership programme".

A small protest was held outside the conference at the TUC's Congress Centre in London by Unison and Unite, the unions representing police civilian staff, who have warned that extending the role of the private sector "is a dangerous experiment with local safety".

Ben Priestley, the Unison national officer for police staff, said that private companies "are accountable only to shareholders, not local citizens".

Lord Prescott, who hopes to be Labour's candidate for police commissioner in Humberside, has launched a "keep policing public" campaign on Twitter.

But the two police forces and authorities involved say that any company eventually working with them would be "expected to share and uphold their values".

The conference is being held as G4S starts a trial in Lincolnshire of its "street to suite" programme under which the private security company picks up suspects arrested by officers on the street.

"The whole process of arresting someone can effectively mean a police officer is off the street for up to an hour and a half," said a G4S spokeswoman.

"Under 'street to suite', the officer calls us and we transport the prisoner to custody using police-style vans." The development is part of a £200m outsourcing deal to run a range of policing services in Lincolnshire. It is already up and running in south Wales.

The West Midlands/Surrey contract is on a different scale and says that some services will be directly managed by the private sector. It says that not all the activities identified in it will be included in the final contract but it also reserves the right to add services incrementally as it progresses.

Both the West Midlands and the Surrey police forces have moved to reassure the public that the tendering process will not result in private security guards patrolling the streets. But the contract does specifically include "patrolling neighbourhoods" among its list of activities.

The Surrey force pioneered the use of community support officers, who do not have the power of arrest, to conduct street patrols.

The Association of Chief Police Officers has stressed that private security firms already routinely patrol public spaces, including for local authority and residents' contracts.

The West Midlands force has confirmed that detaining suspects and "elements of criminal investigations" under the supervision of a police officer are within the scope of the contract.

Chris Sims, the West Midlands chief constable, said: "This is not about taking away the core responsibility for policing away from officers. I want to be clear that where a police power is needed a police officer will carry out that duty.

"The backbone of the service will remain unchanged but we are committed to finding a long-term transformational solution that is more cost-effective and improves the service we deliver," said Sims.

"Our values remain at the heart of the service and our priority is to put the public first in everything we do."

Unison's Ben Priestley branded the privatisation plans as "truly frightening". He added: "People in the West Midlands and Surrey have not had any say in these plans and yet their safety and confidence in the police will be severely jeopardised if this contract goes ahead. It is true that forces are struggling with a 20% cut in their budgets, but privatising policing is not the answer. It will only lead to more cuts, as private companies shed staff and close services to make their profit.

"There is a real danger that creeping privatisation under this government will see private companies running the whole of our justice system."

Peter Allenson, Unite's national organiser for the services sector, said: "Allowing private sector companies to profit from such an important service is deeply worrying. Not just to victims and witnesses of crime but for police staff who are vital to the frontline.

"Privatisation has nothing to do with making our streets safer. It has everything to do with cutting costs and making money. The drive to privatisation almost always leads to the erosion of terms and conditions, attacks on pensions and jobs. We have genuine fears for what this means to the quality of policing. We need to build a coalition of resistance across our communities to stop the government from dismantling our police service."