The measures, which come into force on Jan 1, are the biggest expansion in decades of police powers to deprive people of their liberty.

Hazel Blears

At present, officers can generally arrest people if they suspect them of committing an offence which carries at least five years in prison. They will now have the discretion to detain someone if they suspect any offence and think that an arrest is "necessary".

The civil liberties organisation Liberty said the change represented "a fundamental shift" in power from the public to the police and the state and was open to misuse.

It pointed out that powers to stop people under anti-terrorist legislation, which the public had been reassured would be applied correctly and sparingly, were wrongly used against an elderly heckler at the Labour Party conference in the autumn.

There are also worries that the new arrest laws will create major problems for constables, whose judgment on the "necessity" of an arrest is likely to be routinely challenged in the courts, particularly under human rights legislation.

Officers will have to satisfy themselves of "a person's involvement or suspected involvement or attempted involvement in the commission of a criminal offence" and that there are "reasonable grounds for believing that the person's arrest is necessary".

They will also have the power to take digital photographs of suspects on the street when they have been arrested, detained or given a fixed penalty notice.

The Home Office said the move would save time spent in taking suspects to a police station to be photographed and that it would "greatly reduce the ability of suspects to deny that they were the person in question".

But many people fear that the move will create a vast database of photographs of innocent citizens which could be kept even if the police decide not to take any further action against them.

The Government says that the existing legal framework on arrestable and non-arrestable offences has become "bewilderingly" complex and needs to be simplified.

A Home Office spokesman said yesterday that arrests would not soar because, in addition to the necessity test, many offences would be covered by fixed penalty notices.

Police chiefs have made clear that, although they were concerned about the current system, they did not ask for all offences to be arrestable.

Liberty said that three years ago the Home Office and the Cabinet Office had advocated "a definitive list" of arrestable offences and enhanced training, not a move towards all offences being arrestable.

Mark Oaten, the Liberal Democrat home affairs spokesman, said: "Officers need firm guidance on how to use these new powers. Nobody wants to live in a society in which every offence results in people being dragged down to the police station."

Edward Garnier, the Tories' spokesman on home affairs, said: "The effect of the new arrangements will need to be monitored closely."

Like Liberty, he referred to the ejection from the Labour conference of Walter Wolfgang, 82, a refugee from Nazi Germany and a Labour Party member since 1948, and how a policeman citing the Terrorism Act detained him when he tried to get back into the hall.

Hazel Blears, the Home Office minister, said: "It is vital that the police are equipped with the powers they need to enable them to do their jobs properly and effectively. The powers need to be updated to reflect modern policing priorities and the changing nature of criminal activity.

"We need to maintain the crucial balance between the powers of the police and an individual's rights.

"The introduction of a single, rationalised power of arrest simplifies arrest powers and requires the police officer to consider the necessity of the arrest."