Anti-government demonstrations have rocked the Iraqi capital and its south this month, leaving more than 250 people dead so far -- around half of them in the capital.

Amnesty said an investigation it carried out showed that security forces in Baghdad had deployed military-grade tear gas canisters "to kill rather than disperse protesters".

"All the evidence points to Iraqi security forces deploying these military-grade grenades against protesters in Baghdad, apparently aiming for their heads or bodies at point-blank range," said Amnesty's Lynn Maalouf.

"This has had devastating results, in multiple cases piercing the victims' skulls, resulting in gruesome wounds and death after the grenades embed inside their heads," said Maalouf, Middle East research director.

The rights watchdog said the tear gas grenades being used were two variants from Bulgaria and Serbia, which are "up to 10 times heavier than regular tear gas canisters".

When fired directly at protesters they cause "horrific injuries and deaths", it added.

Demonstrators have told AFP that tear gas canisters deployed during the last week of protests were reaching further, causing more severe asphyxiation and trauma wounds than those used in earlier rallies.

A doctor in Baghdad said it was "the first time" he had seen puncture wounds from tear gas grenades, even after treating casualties from rallies in previous years.

"We can tell they have been hit by the grenades from the smell. If they're still alive, we search for the wound and try to pull out the grenade," the doctor told AFP.

"It's clear that it's a direct hit," he added.

Horrifying footage also circulated on social media showing young men with their eyes, mouths or other body parts smoking after apparently being hit with tear gas canisters.

Amnesty said it had verified several of those videos as well as CAT-scan imagery from medical workers in Baghdad showing entire grenades embedded in the skulls of victims.

It confirmed five deaths due to the grenades in as many days, with military experts, medics and forensic pathologists saying the "horrific nature" of the casualties was "unprecedented".

The Iraqi Human Rights Commission said it had documented 100 deaths since a second wave of protests erupted on October 24, due to tear gas, trauma wounds from the canisters and live ammunition.

An earlier six-day round of rallies was even deadlier, with at least 157 killed, according to an official probe.

A security source told AFP Iraqi police had complained they "were not trained to deal with such large mass protests".

Amnesty urged the police to immediately stop using these weapons.

"What we've documented with these grenades in Baghdad goes far beyond misuse of a 'safer' weapon -- the very design of the grenades being used is maximising the horrific injuries and death," said Maalouf.