Concerns have been raised for public safety as Taser stun guns are made available to more police officers in England and Wales.
Tasers disable a suspect with an electric shock
Officers in 10 forces, who are not firearms specialists, will be able to use the 50,000-volt Tasers to protect themselves or the public.
Amnesty International fears police using the weapons in the one-year trial may not be properly trained.
The Home Office says officers will undergo a rigorous selection procedure.
Until now, about 3,000 Tasers had been issued in Britain, but only to members of police firearms units.
Firearms officers could only use them when confronted by an armed attacker, but their powers were extended in July to include incidents of severe violence or threats.
Now officers from other units will be able to use Tasers, which temporarily disable a suspect.
The Tasers deliver powerful electric shocks and are intended to be a "less lethal" alternative to conventional firearms.
The Home Office said officers would not be routinely equipped with Tasers.
Instead, there would be a selection procedure and only specially trained officers who completed a training programme approved by the Association of Chief Police Officers (Acpo) would be issued with one.
A Home Office spokeswoman said every deployment of a Taser was monitored and a report produced.
Since their introduction in April 2003 to July 2007, Tasers have been used in more than 800 incidents, she said.
Before approving this latest trial, the Home Office ordered a medical assessment of the weapon by a special committee and it found "the risk of death or serious injury from Tasers remains low".
But Amnesty International said stun guns were potentially lethal and there had been numerous deaths in the US because they have been misused.
Spokesman Mike Blakemore said: "The police have a very difficult job to do and they need to protect themselves and they need to protect the public.
"You need trained firearms officers who not only know how to fire a Taser but know when to fire a Taser. These are potentially very dangerous weapons."
He said firearms officers underwent continuous training for real-life situations, with training repeated every month to keep them up to date.
TASER PILOT SCHEME FORCES
Avon and Somerset
Devon and Cornwall
Metropolitan Police Service
"What we do not know is exactly what the nature of the training is that non-firearms officers will receive," he said.
"And we're concerned that it won't be up to the same standard."
Former Scotland Yard commander John O' Connor told BBC News 24 he believed extending the number of officers using Tasers was dangerous because officers were not being properly trained.
He said by giving out Taser guns "indiscriminately to untrained officers" there was a risk they would be used far too indiscriminately.
"What they should be doing is every single recruit that comes in should be firearms trained," he said.
Former chief superintendent of West Midlands Police Barry Mason said giving officers "quick training in taser techniques and using the taser gun was a knee-jerk reaction".
"We need more officers on the streets who are armed with the proper conventional firearm - not every officer, but more officers, armed properly," he said.
Death in Durham
Alan Gordon, vice-chairman of the Police Federation, said his members welcomed an expansion of Taser use and wanted to see the scheme rolled out to all frontline officers.
Wider use of Tasers would bring a sharp reduction in assaults on police officers, he said.
He added that people recover from Taser's side-effects far quicker than if they were subjected to CS spray or a baton.
"I have got confidence in the members that I represent that they will use these weapons appropriately," he said.
In 2006, Brian Loan, 47, died several days after being shot by a Taser in County Durham.
A coroner recorded a verdict of death by natural causes, attributing his death to heart disease, but his sister, Barbara Hodgson, refused to accept the Taser was not to blame and said future cases would prove her right.