Protesters gathered in Cairo on Tuesday for rival rallies over a deeply disputed constitutional referendum proposed by Egypt's Islamist president, Mohammed Morsi, raising fears of street clashes.

Morsi has ordered the army to support police in keeping security during the protests and until results are given from the referendum, which is due to take place on Saturday.

Last week, clashes between the rival camps wielding metal bars, Molotov cocktails and handguns left seven people dead and hundreds injured, and prompted the military to deploy tanks and troops around the presidential palace.

Since the weekend, concrete barriers have been erected outside the palace, one of the main rallying points for the demonstrations.

The pro-Morsi Coalition of Islamist Forces supporting the referendum will start Tuesday's protests.

Their members - including many partisans of Morsi's Muslim Brotherhood - were to gather in the Nasr City district just two kilometers from the palace.

Later, the opposition coalition, the National Salvation Front, was to hold its own rallies in the city's Tahrir Square, the focus of the popular uprising that toppled former president Hosni Mubarak last year, and outside the palace.

Each demonstration is expected to draw tens of thousands of people. Should the opposing rallies meet, there is a high risk of violence.

The fresh protests "raise the specter of 'bloody Wednesday'," the independent newspaper Al-Shuruq headlined, referring to last week's deadly clashes.

The pro-government daily Al-Akhbar said: "My God, save Egypt".

The opposition, made up of secular, left-wing and liberal groups, has vowed to scupper the referendum and the draft constitution, which was approved by an Islamist-leaning panel.

It sees the draft text as weakening human rights, the rights of women and religious minorities.

Late on Monday, 23 Egyptian human rights groups, including the Cairo Institute for Human Rights Studies and the Association for Freedom of Thought and Expression, warned it could put the country on the path to a strict Islamic state like Iran's.

Morsi's supporters argue that it is now up to Egypt's voters to decide the referendum.

The United States called for the protests to remain peaceful.

"We also want to see the Egyptian government and security forces respecting that freedom of peaceful expression and assembly and to exercise restraint," US State Department spokeswoman Victoria Nuland said.

On Monday, after a meeting with Morsi, Egypt's defense minister and commander of the armed forces, General Abel-Fattah al-Sisi, called on army officers to exercise the "highest levels of self-restraint".

He said the armed forces were determined to "carry out their role in protecting the nation and its stability regardless of pressures and challenges".

Agence France-Presse