Friday prayers in Cairo ended with thousands of Egyptians marching to Tahrir Square, where they were joined by a number of prominent politicians, Ahram Online reported.
Demonstrators called on Arab countries to cut ties with Israel and to express unity with Palestinians in Gaza, where Israeli forces have carried out hundreds of air and naval strikes in retaliation for persistent rocket attacks on Israel, originating from Gaza.
Muslim Brotherhood leader Mohamed El-Beltagy and Salafist cleric Safwat Hegazy, joined the demonstrators.
Egyptian Prime Minister Hisham Qandil has called for de-escalation of the conflict.
An Egyptian government official warned a humanitarian crisis could occur in Gaza if clashes lasted long than a week. He called the situation "very volatile."
Karl Schembri, of the global charity group Oxfam, said residents of Gaza are already suffering. Large parts of the region are experiencing power shortages, and the greatest fear is of a fuel shortage, he said.
Humanitarian conditions in the Gaza Strip had worsened before the current clashes, Schembri said, because of the 6-year-old Israeli blockade on the movement of certain goods.
Without an end to the blockade, he said, "reconstruction is impossible."
Arab diplomats at the United Nations recently called on the international community to help improve humanitarian conditions in Gaza.