Abu Sisi went missing on February 19 after boarding a train in the Ukraine during a visit to his wife's family. On Sunday, Israeli media confirmed earlier reports that he was abducted by Israeli officials after the state partially lifted a gag order on reporting the man's disappearance.
The gathered protesters were demanding his release.
The engineer's son, 10-year-old Mousa, held a photo of his father and urged the Ukrainian president to pressure Israel to release him. He said his father was held in Israel's Ashkelon prison "for no offense."
Abu Sisi's sister delivered a speech on behalf of the family refuting Israeli reports that her brother was affiliated with resistance factions in the Gaza Strip.
"The Abu Sisi family condemns this Israeli slander against Palestinian scholars and thinkers," she said.
Protesters also appealed to human rights groups to lobby for Abu Sisi's freedom.
A delegation representing the Gaza Strip's union of engineers joined the sit-in, including union chief Kan'an Ubeid. They raised posters in English and Arabic calling on Ukrainian authorities to intervene and free their colleague.
"Where is my father Dr Dhirar Abu Sisi?" read one sign.
Following the official's disappearance in February, his wife asked the Ukrainian interior ministry for help in establishing his whereabouts, and said she suspected he was kidnapped by Israel's Mossad.
Israeli courts imposed a gag order on the country's media, banning reports on the man's disappearance. After the order was partially lifted, the Israeli newspaper Haaretz confirmed that the man was held in Israel, but said "Much of the rest of the details remain under gag order."
