The medical sources said the protesters were hit near Tahrir (Liberation) Square, the epicentre of the weeks-long movement for sweeping political reform.

The United Nations had already documented 16 deaths from the military-grade canisters, which are up to 10 times heavier than regular tear gas grenades and can pierce skulls or lungs.

Amnesty International found that demonstrators suffered "gruesome wounds and death after the grenades embed inside their heads."

The latest casualties came as the embattled government faces new pressure from the United Nations to enact reforms, including constitutional amendments and changes to the electoral law.

More than 330 people have been killed, most of them protesters, since demonstrations erupted in Baghdad and the Shiite-majority south on October 1.