The National Salvation Front in Egypt, a recently created umbrella group comprised of opposition groups, issued a statement on Thursday in which it said that the "authority" had lost its legitimacy and called for mass protests on Friday.
According to a report on the website of the Egyptian Al-Ahram daily, the statement follows deadly clashes between supporters of President Mohammed Morsi and opposition protesters that left six dead and almost 700 injured on Wednesday.
The clashes erupted after supporters of the president dispersed a sit-in by non-Islamist groups following a mass protest a day earlier.
Hundreds of thousands marched to the presidential palace in Tuesday's demonstration to protest against recent decisions by Morsi they deemed "dictatorial," said the report said.
"The people have noticed an evident abandonment of the duty to protect individuals and assets by the authorities which lost them their legitimacy," the statement read.
The clashes broke out after two weeks of simmering tension due to a November 22 constitutional declaration by Morsi, which gave him immunity from judicial oversight and protected the Constituent Assembly – which drafted a controversial constitution – from dissolution.
The opposition argues that the draft constitution – which will be put before a popular referendum next week after having been written by Egypt's Islamist-led Constituent Assembly – would have an adverse effect revolutionary demands including civil liberties and social benefits.
"We had hoped the president would have responded to repeated demands to annul the constitutional declaration and postpone the constitutional referendum until a national consensus on the draft constitution was reached," it stated.
The Front also rejected invitations to hold dialogue after Wednesday's events, which it had accused the Muslim Brotherhood of inciting in a previous statement.
"Following the latest violent events, it is difficult for the front to negotiate, ignoring the innocent blood spilt," it asserted, saying that the presidency and the government had failed to take the necessary steps and decisions to prevent the bloodshed.
In a speech on Thursday, Morsi said he will not tolerate killings or sabotage but will respect free speech.
"We respect peaceful freedom of speech but will never allow anyone to take part in killings and sabotage," he said.
On Thursday, medics reported, seven people had died in the continuing clashes between Morsi supporters and secular-leaning opponents outside the presidential palace.
The military set up a barbed wire barrier 150 meters from the palace, after ordering Morsi allies and foes alike to pull back.
Morsi vowed to push on with a December 15 referendum on a controversial new constitution, saying "afterwards, there should be no obstacle and everyone must follow its will."
As he was wrapping up his speech, reported AFP, protesters stormed the Cairo villa housing the headquarters of the Muslim Brotherhood which backed him for the presidency, setting it on fire.
"Two hundred thugs went to the headquarters. Security tried to prevent them, but some got through the back door, ransacked it and set it on fire. It is still burning now," Brotherhood spokesman Mahmud Ghozlan said, according to the report.