Attended by more than 200,000 people waving yellow Fatah flags, the rally descended into chaos when gunfire rang out amid what Hamas described as battles with the opposing group's fighters.
Six people, all civilians, died in the exchanges, and 80 others, including several Hamas security men, were injured, according to Dr. Muawiyah Hassanein, head of Gaza's emergency medical services, Reuters reported.
Officials on both sides were quick to blame the other for the first major violence since Hamas seized control of the Gaza Strip more than six months ago. Fatah activists blamed Hamas gunmen for opening fire from the Islamic University in Gaza City. Hamas countered that its men came under attack from Fatah gunmen and returned fire.
Fatah officials, most of whom fled the region once Hamas took control, called on the group's supporters to resist Hamas and to use the clashes as a rallying cry.
"The people came out today into the streets to say 'no' to Hamas," Mohammed Dahlan, a former security chief in the Gaza Strip, told Reuters from the West Bank. "I call on Fatah in Gaza to continue pursuing peaceful means in confronting Hamas. ... Popular means are the only way to bring the downfall of this fascist movement."
The clashes come as Palestinian President Mahmoud Abbas, of Fatah, prepares for a U.S.-sponsored conference with Israel on Palestinian statehood later this month. He has rejected talks with Hamas until the group gives up control of the Gaza Strip.
The U.S.-hosted meeting is tentatively scheduled for Nov. 25-27 in Annapolis, Md., and Abbas and Israeli Prime Minister Ehud Olmert plan to attend.
U.S. Secretary of State Condoleezza Rice has been meeting with both men in an effort to lay the groundwork for an agreement.
The first stage of the 2003 roadmap to peace calls for efforts by the Palestinians to build state institutions and fight terrorism, while Israel halts the growth in West Bank settlements and removes settler outposts considered illegal by Israeli law.
"What we need for a successful meeting in Annapolis is to implement the first phase of the roadmap," said chief Palestinian negotiator Ahmed Qurei, according to the New York Times. "We have suspicions of each other over seven years, so need to build trust."