The demonstrations occurred just hours after the killing of Haniyeh and one of his bodyguards.
Haniyeh was in Tehran to attend the inauguration ceremony of Iranian President Masoud Pezeshkian.
Protests erupted across the capital, the holy city of Mashhad, the central city of Isfahan, the northwestern cities of Tabriz and Ardabil, the northern cities of Sari and Gorgan, the western cities of Hamadan, Zanjan, and Kermanshah, the eastern city of Birjand, and numerous other locations across Iran.
Participants denounced the assassination as an act of terrorism and protested the perpetrators’ violation of the Islamic Republic’s sovereignty.
They chanted slogans against Israel and its allies, including the United States, and demanded a decisive response to the occupying entity.
Protesters considered the assassination an indication of Israel's desperation in the face of the regional Axis of Resistance, including Hamas, which has confronted Tel Aviv's atrocities against Palestinians.
They asserted that the assassination would not deter the Palestinian nation and their allies from defending the Palestinian cause but would strengthen their resolve.
The demonstrations followed a warning from Leader of the Islamic Revolution Ayatollah Seyed Ali Khamenei, who promised a "harsh response" to the assassination, calling it the Islamic Republic's duty to avenge the blood of the resistance leader.