Police in England and Wales are to start carrying a controversial electronic stun gun that will knock out suspects.
The M26 Taser "electro-muscular disruption" gun will be used alongside conventional weapons in London, Thames Valley, Northamptonshire, Lincolnshire and north Wales from Thursday.
Its introduction follows growing pressure for a "less lethal" weapon to reduce the number of people shot dead by armed police.
But Amnesty International say Tasers inflict "intolerable pain" and are easily abused.
And civil rights group Liberty has concerns about the safety of the weapons.
Firearms officers have been ordered to shout: "Taser! Taser! Taser!" before firing at suspects believed to be armed or dangerous.
The £200 gun uses compressed air to fire, at 180 feet (54.86 metres) per second, two needle-tipped darts that trail electric cable back to the handset.
A laser helps target victims up to 21 feet (6.4 metres) away.
When the darts strike, a five-second 50,000-volt charge that will penetrate
clothing up to two inches (five centimetres) thick is released down the cable, causing the suspect's muscles to contract uncontrollably.
That causes temporary paralysis.
Once the shock and pain subside, the victim has just two small marks.
But manufacturers, Arizona-based Taser International, warns they can cause eye injuries when fired towards the face.
The BBC's Daniel Sandford said there had been three deaths in the US linked to the use of Taser, but none were proved.
Hertfordshire Chief Constable Paul Acres, of the Association of Chief Police Officers, told reporters at the launch of the weapons in Northampton there was no direct link between Tasers and death.
He added: "There is a risk
of using it and any weapon but it's a question of taking a responsible approach
to the use of it."
Amnesty International says the full effects have never been independently tested.
Tasers record every firing to prevent misuse.
But there is a government ban on the export of stun guns "because of evidence of their use in torture", according to Amnesty International UK's arms campaigner Robert Parker.
John Wadham, director of Liberty, said the weapons should be medically proven to be safe before they are used on the public.
"If stun guns can safely tackle people who would otherwise be shot, then
that's a welcome improvement.
"But this must only be about reducing the force used by police, not about
"These weapons should only be used where firearms are the only other option."