The British Police Complaints Authority said yesterday it had "reservations" about the use of stun guns, which two police forces are considering issuing to officers.
The Metropolitan Police said it hoped "taser" guns, which temporarily paralyse their victims with electric shocks, could be introduced by the end of the year.
But the PCA said that while it recognised the need to find a weapon less lethal than firearms for the police armoury, it was concerned about proposals to issue tasers to officers dealing with public order offences.
Amnesty International also voiced concerns about the effects of the new weapons, which fire two electrical probes up to 21 ft to shock and temporarily disable suspects.
Deputy chairman of the PCA Mr Ian Bynoe said: "We would see stun guns being used when people are clearly armed with a knife or sword. But we have reservations about any proposals to issue them to officers involved in public order situations.
"The shock delivered is painful and must therefore be delivered for the minimum length of time - literally just a second and no more," he added.
"The Human Rights Act justifies the taking of life by the state only when absolutely necessary and we would therefore support a trial of stun guns as an alternative to the police having to use lethal force," Mr Bynoe said.
An Amnesty spokesman, Mr Brendan Paddy, said: "This device fires darts sharp enough to pierce a leather jacket - if that were to hit you in the face it would cause an extremely debilitating injury.
"But the principal concern is the effect an electric shock could have on someone who already has a medical problem, such as a heart condition.
"The police forces concerned should suspend the plan to issue these weapons until there has been proper, independent, medical investigation."
If the medical risks are found to be acceptably low there should also be proper training and guidelines on use, he added.
A Scotland Yard spokeswoman said the proposal to introduce stun guns was designed to provide officers with more non-lethal deterrents to tackle violent criminals.