(CNN) -- At least seven Palestinians were killed and 55 wounded Monday when Hamas security forces and Fatah gunmen traded fire at a Gaza City rally, according to Palestinian medical sources.
Fatah supporters carry a comrade wounded Monday in clashes with Hamas, a rival Palestinian faction.
The rally was being held to commemorate the third anniversary of Palestinian leader Yasser Arafat's death.
Of the 55 wounded, seven are in critical condition, the sources said. The identities of the dead and wounded were not immediately known.
It is unclear how the incident began; each side blamed the other for firing first. View scenes from the rally, firefight »
The Gaza rally began Sunday, the anniversary of Arafat's death, and ended Monday.
Video footage showed crowds of men running through the street as gunfire erupted. Some protesters were carrying the wounded off the streets and to a waiting ambulance. Watch the clashes erupt »
Hamas, which took control of Gaza in a violent mid-June siege, sent members of its executive force to enforce security. Hamas security sources said 130 Palestinians were arrested in the wake of Monday's violence.
Violence had been expected at the rally, which marked the largest Fatah gathering since Hamas took over the area.