Egyptian police and anti-government protesters clashed for a fifth day in Cairo Wednesday as the United Nations condemned the use of excessive force against the demonstrators.

A human rights watch group said at least 38 people have been killed since Saturday.

In a national address, Field Marshal Hussein Tantawi, Egypt's military ruler, promised that presidential elections would be held in the first half of 2012, but he did not specify a date.

Tens of thousands of protesters in Tahrir Square responded by chanting -- "Leave! Leave!"

Protesters across the country are demanding the military council, which took over for ousted President Hosni Mubarak, step down. They want a civilian council to rule the country until elections for a new parliament and president are held.

Navi Pillay, the U.N. high commissioner for human rights, denounced efforts by Egyptian security forces to suppress the demonstrations.

"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," Pillay 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," she said. 

"The actions of the military and police were inflaming the situation, prompting more people to join the protests," she said. "The more they see fellow protesters being carted away in ambulances, the more determined and energized they become."