By Tom Perry and Yusri Mohamed CAIRO/ISMAILIA (Reuters) - Opponents of Egyptian President Mohamed Mursi planned mass demonstrations on Friday, raising the prospect of more bloodshed despite a pledge by politicians to back off after the deadliest week of his seven months in office. Protests marking the second anniversary of the uprising that toppled Hosni Mubarak have killed nearly 60 people since January 25, prompting the head of the army to warn this week that the state was on the verge of collapse. The country's most influential Islamic scholar hauled in rival political leaders for crisis talks on Thursday and persuaded them to sign up to a charter disavowing violence and committing to dialogue as the only way to end the crisis. But barely had those talks at a mediaeval university ended, when Mursi's foes called for new nationwide protests, including a march on the presidential palace in Cairo, which his followers see as a provocative assault on a symbol of his legitimacy. In a statement released overnight, leftist leader Hamdeen Sabahi said signing the peace initiative did not mean an end to the protests. He would not enter dialogue until bloodshed was halted, the state of emergency lifted and those responsible for the previous week's violence brought to justice. "Our aim ... is to complete the goals of the glorious January revolution: bread, freedom and social justice," he said. The protesters accuse Mursi of betraying the spirit of the revolution by concentrating too much power in his own hands and those of his Muslim Brotherhood, a decades-old Islamist movement that was banned under Mubarak. The Brotherhood accuses Mursi's opponents of trying to bring down Egypt's first democratically elected leader and to seize power through street unrest that they could not win through the ballot box. The rise of Mursi, an elected Islamist, after generations of rule by authoritarian, secular military men in the most populous Arab state, is probably the single most important change of the past two years of Arab popular revolts. But seven months since taking power after a narrow election victory over a former general, Mursi has failed to unite Egyptians and protests have made the country seem all but ungovernable. Cairo's streets were still quiet in the morning, with protesters expected to gather following afternoon prayers. The violence has been worst in cities along the Suez Canal, especially Port Said, where demonstrators were enraged by death sentences handed down against 21 soccer fans on Saturday for stadium riots a year ago that killed more than 70 people. Dozens of protesters were shot dead in Port Said and Mursi responded by imposing a state of emergency and curfew there and on two other Suez Canal cities. Protesters plan to demonstrate at the stadium on Friday, the first anniversary of the riots, said Mahmoud Naguib, an activist in the April 6 movement. They also plan other marches after midnight to defy the curfew. "We have one demand: that Mursi and the interior minister go on trial for inciting the killing of protesters in Port Said," Naguib said. Brotherhood supporters battled protesters outside the presidential palace during protests against Mursi in December. Critics accused the Brotherhood of deploying a militia against the demonstrators. Keen to avoid a repeat, the Islamists have so far kept off the streets during the latest wave of protests. Thursday's meeting of political leaders was convened by Sheikh Ahmed al-Tayyeb, head of the 1,000-year-old al-Azhar university and mosque, one of the few institutions still seen as neutral in a society that is increasingly polarized. That marked a climbdown by Mursi's foes who had previously rebuffed invitations to negotiate, demanding that Mursi first promise to include them in a unity government. The presidency said the initiative was "an important step on the road to achieving stability in the Egyptian street." But it is far from clear that opposition politicians could call off the street demonstrations, even if they wanted to. The protest movement has become a spontaneous expression of anger, often only loosely allied to the secularist and liberal parties running against the Brotherhood in elections. "You have groups who clearly just want a confrontation with the state - straightforward anarchy; you've got people who supported the original ideals of the revolution and feel those ideals have been betrayed," said a diplomat. And then you have elements of the old regime who have it in their interests to foster insecurity and instability. Additional reporting by Yusri Mohamed in Ismailia; Writing by Peter Graff; Editing by Paul Taylor