Hamas has rounded up scores of Fatah activists in the Gaza Strip following a Monday rally that drew more than 200,000 supporters and ended in gunfire that killed seven people, officials said on Tuesday.

Islam Shahwan, spokesman of the Hamas-led Executive Force, said it had detained about 50 Fatah members since the rally.

"They are the ones who planned and organised the rally yesterday and are suspected of being responsible for the chaos that took place," Shahwan said.

Fatah official Hazem Abu Shanab said Hamas security forces arrested 400 Fatah members and dozens more were ordered to report to police stations for questioning.

At least 55 people wounded in exchanges of fire between Hamas forces and Fatah supporters, in the worst interfactional fighting in Gaza since Hamas violently seized the Strip in June.

All of the fatalities were either Fatah members or passersby. Palestinian Authority Chairman Mahmoud Abbas blamed Hamas, terming its actions "an abominable crime."

Gaza City was quiet and tense the morning after the violence, with few Hamas policemen in evidence on the streets. Schools were closed in mourning.

In an interview Monday with The Associated Press, Fatah's former strongman in the Strip said the rally showed that Hamas was losing its grip on power.

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]."

The clashes erupted after Hamas policemen tried to disperse a memorial march in Gaza City marking the third anniversary of Yasser Arafat's death. The rally, the largest ever held in the Gaza Strip, attracted more than 200,000 people, who waved pictures of Arafat, yellow Fatah flags, and black and white kaffiyehs as they listened to speeches by several Fatah leaders.

IDF, PA sources: Hamas' most extreme faction now in control of Gaza

Hamas' most extreme faction is now effectively in control of the Gaza Strip, Palestinian and Israel Defense Forces sources have told Haaretz.

The Gaza sources said Monday that the rift within Hamas' leadership is worsening, and the more extreme faction has taken over the organization in the Strip.

The sources said the faction includes most of Hamas' military wing, and is led by former foreign minister Mahmoud a-Zahar, head of the military wing Ahmad Jabari, and former interior minister Sa'id Siyam.

It relies on Iranian funding, the sources said, and is conducting a completely independent policy: It no longer listens to either Khaled Meshal, head of the organization's Damascus-based political office, or Hamas prime minister Ismail Haniyeh.

The sources estimated that Haniyeh controls at most 30 percent of Hamas' fighting forces - even among the Executive Force, which is technically subordinate to his government.

Senior IDF officers offered a similar assessment. The military wing, controlled by Jabari, has effectively gained control of Gaza, they said; this faction ignores directives from Meshal and dictates to Haniyeh.

The extremist faction seeks to reproduce Hamas' takeover of Gaza in the West Bank. Haniyeh's faction, in contrast, believes the takeover of Gaza was a mistake, and has no desire to repeat it in the West Bank.