Supporters of the Muslim Brotherhood and its Islamist allies massed outside the Rabaa al-Adawiya mosque in Cairo's Nasr district to demonstrate their backing for Morsi in his rejection of opposition calls to step down just a year into his term of office. They gathered under the slogan "legitimacy is a red line", in reference to Morsi's insistence that he won a free and fair election and has a popular mandate. Opponents of the Islamist president gathered outside Cairo's Al-Azhar -- Sunni Islam's highest seat of learning -- for a march to Tahrir Square, the iconic epicenter of the protest movement that ousted veteran strongman Hosni Mubarak in 2011. Hundreds of Morsi opponents have been holding a sit-in in Tahrir since Tuesday, June 25. Their protest was called by the Tamarod movement (Arabic for rebellion) which says it has collected more than 15 million signatures to a petition demanding Morsi's resignation and a snap election.