At least three people have reportedly been killed in a gunfight between demonstrators and the Egyptian army. Shots were fired as supporters of Mohamed Morsi marched on the Cairo headquarters of the Republican Guard, news agency AFP said.

Thousands of people streamed towards the military barracks after a Muslim Brotherhood rally at the city's Rabea al Adaweya mosque. The deaths came after the United Nations' human rights chief expressed concern at the arrest of key members of Egypt's Muslim Brotherhood.

Navi Pillay said there should be "no more violence, no arbitrary detention, no illegal acts of retribution" following the military intervention that forced Mr. Morsi from office. He is believed to have been placed under house arrest since Egypt's top judge, Adli Mansour, was sworn in as interim president.

At least four of his aides and advisers are also being held.

Elsewhere, the Egyptian army positioned its tanks and troops outside the presidential palace in Cairo, blocking access to protesters. It came after the Muslim Brotherhood and its allies called for protesters to take to the streets on the Muslim day of prayer for what it described as a "Friday of Rage".

Egyptians are braced for more violence, although Muslim Brotherhood officials have urged their followers to keep their protests peaceful.

Rise in sexual violence against women

Reporter Andrew Wilson, inCairo, said "Everyone's trying to read the signals and figure out what's most likely to happen next, and they're hoping for some sort of resolution. One thing we will not see this time round, as we might have seen during some parts of the original Arab Spring, is two sides clashing".

"The protesters in Tahrir Square - this so-called coalition or rebellion - all agreed on one point and one point only. They wanted Mr. Morsi out and they've won. They have no interest whatsoever in getting involved in clashes in the streets. Two-and-a-half years ago, it was pro-Hosni Mubarak supporters fighting with anti-Mubarak campaigners, clashing with the army in the middle.

"The army has delivered what half the population wants and now only has the job of policing the other half" he added.

Meanwhile, Ms. Pillay called on authorities to investigate the "appalling" rise in sexual violence against women. It comes after a number of women told Human Rights Watch they had been assaulted in Tahrir Square.

The charity said some women had been beaten with metal chairs, chains and sticks, while others were attacked with knives.