The UN human rights chief called Wednesday for an independent probe into the killing of demonstrators by Egypt's military and security forces.

"I urge the Egyptian authorities to end the clearly excessive use of force against protesters in Tahrir Square and elsewhere in the country, including the apparent improper use of tear gas, rubber bullets and live ammunition," said Navi Pillay, UN High Commissioner for Human Rights.

"Some of the images coming out of Tahrir, including the brutal beating of already subdued protesters, are deeply shocking, as are the reports of unarmed protesters being shot in the head," she said.

"There should be a prompt, impartial and independent investigation, and accountability for those found responsible for the abuses that have taken place should be ensured," the UN rights chief added.

Pillay also castigated the military and security forces for their actions which "simply served to inflame the situation," instead of calming the crowds.

Thousands of people rallied again Wednesday in Tahrir Square demanding an end to military rule, despite a promise by Egypt's interim leader to transfer power to an elected president by mid-2012.

According to the health ministry, 31 people have been killed since Saturday—28 in Tahrir— when the security forces first resorted to tear gas, rubber bullets and birdshot in a bid to break up the demonstrations.

Pillay reiterated her concerns at attempts by the authorities to curb civil society activities.

It is "imperative that the Egyptian authorities ensure the respect of freedom of expression, assembly and association, and of the press" during the electoral process, she stressed.

"The Egyptian authorities have an obligation to provide protection for all and ensure a peaceful and safe environment in the lead-up to next week's crucial elections," the UN High Commissioner said.

"The people of Egypt deserve to exercise their right to vote in the country’s first elections since the departure of former president Mubarak in a violence-free environment," she added.

The latest mass protests resulted in the resignation of the cabinet on Monday, just a week before crucial legislative polls, the first since Hosni Mubarak was ousted.