Egyptian President Adly Mansour has vowed to battle for security "to the end", as the military warns it would clamp down on any violence in rallies planned by Mohamed Morsi's supporters and opponents.
"We are at a decisive moment in Egypt's history, which some want to steer into the unknown," Mansour, installed as interim leader by the military after Morsi's overthrow on July 3, said in a televised address on Thursday.
Morsi's Muslim Brotherhood has refused to recognise Mansour's government, instead placing its hopes in sustained protests it believes may reverse the coup that toppled the elected president after nationwide demonstrations against him.
Although mostly peaceful, Brotherhood rallies have led to clashes that have killed dozens of people since Morsi's ouster.
"We will fight the battle for security to the end. We will preserve the revolution," said Mansour, who was a top judge before becoming president.
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Mansour's speech came as the military deployed reinforcements to the restive Sinai peninsula, where suspected militants killed a policeman on Thursday.
Several policemen and soldiers have been killed in drive-by shootings and rocket attacks since Morsi's overthrow.
In his speech, Mansour again offered an olive branch to Morsi's Muslim Brotherhood.
But he also pledged "transitional justice", amid calls for the prosecution of Morsi and a crackdown on the Islamists.
"The framework of justice and reconciliation extends to all," he said.
Morsi has been detained by the military in a secret location since his ouster.
The Brotherhood and anti-Morsi groups have called for new rallies on Friday, prompting the military to issue a stern warning.
"The armed forces warn against any deviation from peaceful expressions of opinion, and the resort to violence," it said in a statement on its Facebook page.
"Whoever resorts to violence in Friday's protests will endanger his life, and will be treated with utmost decisiveness, within legal bounds," the statement added.
Brotherhood officials told AFP that they had insisted on Morsi's reinstatement in a meeting with EU foreign policy chief Catherine Ashton, who visited Cairo on Wednesday.
It was "impossible to get engaged in the political process under the rules of a military coup", one official, Amr Darrag, said he told Ashton.
"People are increasing on the street," Darrag said of Brotherhood-led protests demanding Morsi's reinstatement.
"The military will definitely keep making mistakes, and definitely more people will come to our side," he added.
The Brotherhood says Morsi could call for early elections if reinstated. The interim government dismisses the proposal as unrealistic.
