Navi Pillay also said that Egyptian President Hosni Mubarak should heed the voice of his people, many of whom have demanded that he step down immediately.

Tens of thousands of Egyptians gathered in Cairo’s Liberation Square on Friday to press for an end to Mubarak’s 30-year rule, hoping their numbers would swell to a million in what they called the “Day of Departure.”

Despite violent clashes between pro-Mubarak supporters and protesters earlier this week, police had been noticeably absent and the army had failed to separate the rival groups, with tragic consequences, Pillay said.

“One of the prime drivers of this chaos seems to have been the actions of Egypt’s security and intelligence services,” she told a news conference. “Change is coming to Egypt, as it came to Tunisia, but the violence and bloodshed must stop now.”

The UN estimates that 300 people have died in the unrest.

Pillay urged authorities to give clear orders to “security and intelligence forces that have protected the authoritarian regime in Egypt for the past 30 years, to stop undermining the security of the state they are supposed to serve.”

She called for an independent investigation into the violence and whether it was planned, and if so by whom, saying her office stood ready to assist in the inquiry.

Pillay also denounced assaults on and detention of dozens of journalists in what she called “clearly a blatant attempt to stifle news of what is going on in Egypt.” At least 20 human rights activists had also been rounded up and detained.

“I also urge the authorities to maintain open communications and Internet services, protect media premises and halt all activities aimed at restricting or manipulating the free flow of information, such as the extraordinary hijacking of Vodafone’s system in order to send propaganda text messages,” she said.

Vodafone, the world’s biggest mobile operator by revenue, accused Egyptian authorities on Thursday of using its network to send pro-government text messages to subscribers.

Mubarak says he is willing to retire but, having portrayed himself as a bulwark against radical Islam in the most populous Arab state, has warned of chaos if he goes now.

Pillay, asked whether he should step down, replied: “It’s not his resignation that will bring on chaos ... There’s chaos going on right now.”

People had gathered again for an 11th day in Cairo, she said, adding: “They are making very strident demands about the way they want the chaos ended.”

“This is a matter the Egyptian people have to decide themselves,” she said. “We endorse the calls from the street for a change to meaningful democracy. I think (that) is where we stand on the matter.”

On Friday, troops were out in force providing security for the demonstrators. Police have been officially redeployed to the streets but have kept a low profile since they were withdrawn after losing control on Jan 28.