Egypt is bracing for rival protests in Cairo on Tuesday over a bitterly divisive referendum on a new constitution, prompting President Mohamed Morsi to order the army to help "preserve security".
The duelling demonstrations, organised by Islamists backing Morsi and the largely secular opposition, raised fears of street clashes like ones last week in which seven people were killed and hundreds injured.
Morsi's decree instructs the military to fully cooperate with police "to preserve security and protect vital state institutions for a temporary period, up to the announcement of the results from the referendum."
Army officers "all have powers of legal arrest," it says.
The military, which has urged dialogue and warned it "will not allow" the political crisis to deteriorate, has for several days kept tanks and troops deployed around Morsi's presidential palace.
Late Monday, soldiers watched without intervening as more than 100 anti-Morsi demonstrators milled around in front of the palace.
The rights group Amnesty International called Morsi's security decree "a dangerous loophole which may well lead to the military trial of civilians."
The group said the measure recalled the 16 months of army rule that followed the ouster of former president Hosni Mubarak last year, until Morsi's election in June 2012.
The opposition, made up of secular, liberal, leftwing and Christian groups, has said it will escalate protests to scupper the referendum.
It views the new constitution largely drawn up by Morsi's Islamist allies, including some who want Sharia law, as undermining secular traditions, human and gender rights, and the independence of the judiciary.
The Egyptian Initiative for Personal Rights said Monday that the draft charter "contains no reference to human rights treaties and conventions ratified by Egypt, reflecting ... disdain for these agreements."
Morsi has defiantly pushed on with the draft, seeing it as necessary to secure democratic reform in the wake of Mubarak's 30-year autocratic rule.
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The main opposition group, the National Salvation Front, has called for huge protests in Cairo to reject the constitutional referendum, which is scheduled for Saturday.
"We do not recognise the draft constitution because it does not represent the Egyptian people," National Salvation Front spokesman Sameh Ashour told a news conference on Sunday.
The Muslim Brotherhood, from which Morsi hails, said Islamist movements would counter the protests with their own big rallies in the capital in support of the referendum.
"We are calling for a demonstration Tuesday, under the slogan 'Yes to legitimacy'," the Brotherhood's spokesman, Mahmud Ghozlan, told AFP.
Morsi's camp argues it is up to the people to accept or reject the draft constitution.
The United States called for peaceful protests and restraint by those charged with maintaining security.
"We want to see those exercising their right of freedom of expression to do so peacefully, but we also want to see the Egyptian government and security forces respecting that freedom of peaceful expression and assembly and to exercise restraint," State Department spokeswoman Victoria Nuland said.
"Again, the way this goes forward has to be worked out among Egyptians ... and of course we don't want to see mistakes of the Mubarak era repeated."
Cairo schools informed parents they would be closed as a precaution on Tuesday.
A group of senior judges on Monday said pro-Morsi Islamist protesters would have to lift a week-long sit-in outside the constitutional court before they would consider overseeing the referendum.
If the charter is rejected, Morsi has promised to have a new one drawn up by 100 officials chosen directly by the public rather than appointed by the Islamist-dominated parliament.
But analysts said still-strong public support for Morsi and the Brotherhood's proven ability to mobilise at grassroots level would likely help the draft constitution be adopted.
If that happens, warned Eric Trager, analyst at the Washington Institute for Near East Policy, it would "set up the country for prolonged instability."