As tens of thousands of supporters of ousted president Mohamed Morsi continue their sit-in at Rabaa Al-Adawiya Mosque in Nasr City, several marches were held in the capital – and in different parts of the country – to mourn the dozens of pro-Morsi protesters killed Monday outside Presidential Guard headquarters in Cairo.

Thousands marched across Nasr City and Giza to protest the incident, which left more than 50 pro-Morsi protesters dead following clashes with military personnel.

In Mansoura, regional capital of Egypt's Daqahliya governorate, thousands marched to mourn the death of student Anas Hamdan, killed in Monday's clashes. Mourners carried banners reading, 'Martyr for legitimacy.'

Demonstrators demanded that the perpetrators be held accountable.  

Hundreds in Alexandria, too, marched through the coastal city mourning slain pro-Morsi demonstrator Mohamed El-Tohamy. Mourners carried El-Tohamy's body to a cemetery east of Alexandria.

El-Tohamy was a Muslim Brotherhood member and a student at Alexandria University's commerce faculty.

In Ismailia, meanwhile, hundreds marched from the Saleheen Mosque bearing coffins and images of slain Morsi supporters.

Clashes erupted early Monday morning between Morsi supporters and military personnel outside Presidential Guard headquarters in Cairo, leaving at least 51 civilians dead and 435 injured, according to health ministry estimates.

The Egyptian army, which claimed it had only attacked the sit-in after an "armed group" had attempted to storm the headquarters, said that one officer died and 42 soldiers were injured in the melee.

Earlier on Tuesday, supporters of deposed president Morsi announced that they would begin a million-man march for the "martyrs" starting at noon.

The marches were called for by the pro-Morsi National Alliance in Support of Legitimacy, an Islamist coalition led by the Muslim Brotherhood, who fielded Morsi for president in last year's elections.

Protesters demanding Morsi's reinstatement as president, meanwhile, have maintained a sit-in for 11 days outside Cairo's Rabaa Al-Adawiya Mosque.

The Muslim Brotherhood, for its part, has reiterated its rejection of the new president. On Tuesday, the Islamist group released a statement describing Mansour as Egypt's "alleged president," declaring the latter's constitutional declaration "illegitimate."