Praising the popular revolt in Egypt, Navi Pillay, the UN high commissioner for human rights, said she was deeply alarmed by the sharp rise in casualties over the past few days.
"Casualties have been mounting on a daily basis, with unconfirmed reports suggesting as many as 300 people may have been killed so far, more than 3 000 injured and hundreds arrested," a statement from Pillay read.
Thousands of protesters thronged the streets around Cairo's central Tahrir Square on Tuesday for the eighth day of anti-government demonstrations.
Organisers of the protest said they were hoping a million people would turn out across the country, and plan a march to the presidential palace in Cairo, the residence of President Hosny Mubarak.
Protesters are calling for democratic and economic reforms and the ouster of Mubarak, who has ruled for nearly 30 years.
"With a million people expected to be out on the streets on Tuesday, I urge both the army and the police to act with the utmost care and restraint," Pillay said.
"Tuesday's march seems likely to be a pivotal moment in Egypt's transition to a freer, fairer and more democratic society."