UN human rights chief Navi Pillay has condemned the "clearly excessive use of force" by Egypt's security forces during clashes with protesters.
Ms Pillay called for an independent inquiry into the deaths of at least 35 people since the weekend.
Protesters are still occupying Cairo's Tahrir Square despite the pledge of a speedier handover to civilian rule.
Security forces again fired tear gas at protesters near the interior ministry after a brief truce collapsed.
Protesters have derided Tuesday's announcement by Egypt's military rulers that presidential elections will be brought forward.
The military is overseeing a transition to democracy following the ousting of President Hosni Mubarak in February. However, many Egyptians fear the army is trying to hold on to power.
Ms Pillay, the UN High Commissioner for Human Rights, said in a statement: "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.
"Some of the images coming out of Tahrir, including the brutal beating of already subdued protesters, are deeply shocking," she added.
"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," Ms Pillay said.
Wednesday's street battles in Cairo again focused on the interior ministry building, near Tahrir Square.
The BBC's Yolande Knell says clashes subsided for a couple of hours after the leader of a local mosque led protesters towards the ministry to shake hands with police officers in a gesture of reconciliation.
State TV reported that police also pulled back to leave troops on the streets.
It is unclear how the truce was broken but chaotic scenes then ensued with a cloud of tear gas and fires at the edge of Tahrir Square, our correspondent adds. Ambulances rushed to treat those injured.
Security forces have been using tear gas and rubber bullets against demonstrators.
There have also been clashes in other Egyptian cities including Alexandria, Suez, Port Said and Aswan.
'Mubarak copy pasted'
The health ministry said on Wednesday that 35 people had died in clashes since Saturday - all but four in Cairo.
State news agency Mena reported that one person had been shot dead in the north-western city of Mersa Matruh as demonstrators tried to storm a police station.
Many protesters camping on Tahrir Square have been wearing safety masks and goggles to protect themselves against the effect of tear gas.
The demonstrators are continuing to demand that the ruling Supreme Council of the Armed Forces (Scaf) hand over power immediately to an interim civilian council.
On Tuesday the head of Scaf, Field Marshal Mohamed Hussein Tantawi, said parliamentary polls scheduled to begin on 28 November would be held as planned, and added that presidential elections would be held by July 2012.
The council had previously said they might not happen until late 2012 or 2013.
Anger at the delayed presidential elections, coupled with a draft constitution produced earlier in the month that would exempt the military and its budget from civilian oversight, prompted the mass demonstrations in Tahrir Square that began on Friday.
The violence is the worst since a wave of street protests toppled President Hosni Mubarak in February, after three decades in power.
After Field Marshal Tantawi spoke, protesters in Tahrir Square chanted: "We are not leaving, he (Tantawi) leaves."
One protester told AFP news agency: "Tantawi is Mubarak, copy pasted. He's Mubarak in a military uniform."
However correspondents say the main opposition group, the Muslim Brotherhood, as well as many Egyptians who are not taking part in protests, seem satisfied with the concessions announced by Field Marshal Tantawi.