"The people want the liberation of Palestine," and "Forward Hamas, you fight for our honour," were among the slogans shouted by demonstrators, who waved Tunisian and Palestinian flags.
The green and white flag of Hamas, the Islamist movement that controls Gaza, was also visible.
Some protesters shouted slogans against Egypt's President Abdel Fattah al-Sisi, whom Islamists have vilified for crushing the Muslim Brotherhood and taking a back seat as Israel pounds Hamas.
"The world, with all its organisations and institutions, watches this odious crime without demanding whether it is more than just a cry of terror," said Nabil Boulifi, a civil servant attending the rally.
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Former health minister Abdellatif Mekki, a senior member of Ennahda, the Islamist party that headed the ruling coalition until its forced resignation in January, welcomed the firing of Palestinian rockets into Israel.
The Palestinians had, he said, "introduced a new equation by responding to the aggression with strikes... against the fortress of the enemy".
Israel's latest air assault on Gaza, launched on Tuesday in response to continuing rocket fire and dubbed Operation Protective Edge, is the bloodiest against the coastal enclave since November 2012.
So far, it has killed 103 Palestinians, including many women and children, and wounded hundreds.
A similar protest was planned in the Moroccan capital Rabat later on Friday.