Officials of the Fatah faction said Tuesday that hundreds of its members were detained by Hamas after deadly violence marred a massive rally in the Gaza Strip a day earlier.

Fatah leaders said a wave of arrests in Gaza targeted activists, including ranking party figures who had organized the rally marking the third anniversary of Yasser Arafat's death. The gathering erupted in gunfire, leaving seven people dead and dozens injured.

The radical Hamas movement has controlled Gaza since its forces in June overwhelmed fighters from the Fatah faction, which was founded by Arafat and one of whose leaders is Palestinian Authority President Mahmoud Abbas.

Hazem abu Shanab, a Fatah spokesman in Gaza, said Hamas security officers detained more than 400 party members and seized computers and documents during raids.

Fatah officials accused Hamas of opening fire on the crowd, which numbered in the tens of thousands -- Fatah's biggest gathering since losing control of Gaza.

Hamas officials said the rival faction's gunmen fired first from neighboring buildings.

Hamas officials accused the rally's organizers of stoking public disorder and said they were arresting those responsible. In a brief statement, the Hamas-run Interior Ministry in Gaza denied that the arrests were politically motivated.

The statement did not specify a number, but the Reuters news agency quoted Islam Shahwan, a spokesman for the Hamas-led Executive Force militia, as saying security officers had detained about 50 people.

The arrests began Monday during the midday rally and continued overnight.

Abbas declared three days of mourning for victims of the fighting and ordered flags flown at half-staff on government buildings.

Abbas decried what he characterized as a Hamas attack and designated the Fatah victims as "martyrs on the road to freedom, independence and the establishment of an independent Palestinian state."

Relations between the two factions have been bitter since the June fighting, which in effect limited Abbas' authority to the West Bank.

Hamas has cracked down on smaller Fatah rallies and has accused Abbas' forces in the West Bank of rounding up Hamas members.

Abbas, who is supported by the United States, has sought to isolate Hamas as he pursues peace talks with Israeli Prime Minister Ehud Olmert.

The two leaders are to attend a U.S.-sponsored peace conference, tentatively scheduled for this month, that is envisioned as a way to revive negotiations aimed at ending the Middle East conflict.