Hamas security forces moved swiftly against their Fatah rivals in the aftermath of a mass Fatah rally that ended with seven people dead, rounding up 400 people in an overnight crackdown, Fatah officials said Tuesday. Meanwhile, former Gaza strongman Mohammed Dahlan said the rally and Hamas's violent response signal that the Islamic terror group's grip is weakening. In Monday's demonstration, more than 250,000 Fatah supporters gathered in a Gaza City square to mark the Nov. 11, 2004 death of iconic Palestinian leader Yasser Arafat. The rally was largely peaceful, but ended in mayhem, with Hamas police opening fire, Fatah protesters throwing stones and thousands of panicked demonstrators running for cover. Seven civilians were killed, and 85 people were wounded, medical officials said. The arrests overnight included dozens of the rally's organizers, Fatah spokesman Hazem Abu Shanab said. Hamas officials were not immediately available for comment. Gaza City was quiet and tense Tuesday morning, with few Hamas policemen on the streets. Schools were closed, and most of the stores in downtown Gaza were shut. Four funerals for victims were planned later Tuesday. On Monday, a funeral for one 19-year-old killed at the rally turned violent when mourners clashed with Hamas men, leaving three people wounded. In an interview Monday, Dahlan portrayed the rally as a sign of the growing discontent of ordinary Gazans with Hamas, which seized control of Gaza by force in June, ousting the security forces loyal to Fatah's leader, Palestinian President Mahmoud Abbas. Dahlan called for more rallies and said Monday's march "will shorten the suffering of the Palestinian people and will shorten the life of this bloody movement (Hamas)." For years, Dahlan has gone head-to-head with Hamas leaders in Gaza, trading bitter accusations. He said he would work closely with Fatah activists, especially in Gaza, "to expose the reality of the Hamas movement ... and move from this dark period to a more promising future." The small terrorist group, Islamic Jihad, which has clashed occasionally with Hamas but shares its ideology, condemned Hamas for the violence. "Despite all of the political differences...it's forbidden and taboo to open fire randomly on a mass popular demonstration," the group said Tuesday.