Underscoring the risk of more bloodshed, several thousand supporters of ousted President Mohamed Morsi's Muslim Brotherhood threatened to march on the military's intelligence headquarters in defiance of a warning from the army to stay away.
They turned back early in the morning, having left the site of a Brotherhood vigil in northern Cairo chanting, "Our blood and souls we sacrifice for Morsi."
The dawn killings on Saturday, following a day of rival mass rallies, deepened the turmoil plaguing the country since the army shunted Egypt's first freely elected president from power on 3 July.
The West is increasingly concerned about the risk of broader conflict in the Arab world's most populous country, a bridge between the Middle East and Africa and recipient of €700 million of economic aid from the EU and more than $1 billion in military aid from the United States.
Catherine Ashton, the EU's foreign policy chief, was scheduled to meet General Abdel Fattah al-Sisi, the head of the Egyptian armed forces who led the overthrow of Morsi, the country's interim president, Adli Mansour, and officials of the Freedom and Justice Party, the Brotherhood's political wing.
Ashton, in a statement, said she would press for a "fully inclusive transition process, taking in all political groups, including the Muslim Brotherhood."
Thousands of Brotherhood supporters have been staging a weeks-long vigil outside the Rabaa al-Adawiya mosque in northern Cairo to demand Morsi's reinstatement, defying threats by Egypt's army-installed authorities to disperse them.
Egypt's devoted son
Sisi, who was appointed by Morsi only to turn against him after a year into the president's rule, made his first appearance since the killings, smiling before television cameras at a graduation ceremony for police recruits dressed in starched white uniforms on Sunday.
He received a standing ovation and was hailed by Interior Minister Mohamed Ibrahim as "Egypt's devoted son."
Fawning coverage in state and private media reflected Sisi's rising political star in a country ruled by former military officers for six decades before a 2011 popular uprising toppled veteran autocrat Hosni Mubarak.
The military says it does not want to retain power and aims to hand over to full civilian rule with a "road map" to parliamentary elections in about six months. But the very public role of Sisi as face of the new order has sown doubt in the army's intentions, and the Brotherhood says it wants nothing to do with his road map.