RNA - The supporters waved Fatah’s yellow flags and held portrait of Abbas and late Palestinian leader Yasser Arafat.
Earlier, the interior ministry in the Hamas-run Gaza granted permission for the festivities in a rare move, saying it is for Fatah to strive for the unity of the Palestinian people.
"We consented to holding these festivities on Al-Wehda Street for Fatah to champion the unity of the Palestinian people," a Hamas leader told AFP, requesting anonymity.
The two movements have been at odds for years, including their recent division over the election. Hamas called for presidential decree setting the timetable for the next vote but Abbas said he wants Israel to first guarantee that the election can take place in east Jerusalem al-Quds.
Addressing the Gaza celebrations on Wednesday, Abbas said Jerusalem al-Quds is the capital of the Palestinian state and is not up for sale or negotiation.
Fatah itself marked the 55th anniversary of its establishment in the West Bank city of Ramallah on Tuesday evening.
The celebration was held near the tomb of late Palestinian leader Yasser Arafat, founder of the faction, with the participation of senior officials of the faction and the Palestinian Authority.
President Abbas said the Palestinian revolution, which started in 1965, is still going on until today "and will continue until achieving victory."
“Many expected that we will finish on the first day or the first year, and many conspiracies had been plotted against us, but instead of being finished, we found ourselves stronger than before,” Abbas said.
Meanwhile, Israeli military forces arrested two distinguished officials from the Fatah movement during separate operations across the occupied Jerusalem al-Quds.
Local sources, requesting not to be named, told Palestine's official WAFA news agency that Israeli forces also arrested the Secretary of the Fatah movement in Jerusalem al-Quds, Shadi Mutawar, in Wadi al-Joz neighborhood of East Jerusalem al-Quds on Wednesday noon.