Palestinian security forces trying to break up a brawl between thousands of Hamas and Fatah supporters opened fire Tuesday, wounding three students in the most violent clash between the rival groups in the West Bank since the fall of Gaza to Hamas last month. The clash erupted at An Najah University, the West Bank's largest, in the city of Nablus. It began when Hamas supporters staged a sit-in on campus and raised their movement's green flags. Fatah activists demanded that the Hamas flags be removed, and a fistfight erupted. Witnesses said students also threw chairs at each other. At some point, dozens of Palestinian security officers broke into the university and opened fire, the witnesses said.