Hamas rounded up dozens of Fatah activists in the Islamist-ruled Gaza Strip after its secular rival drew more than 200,000 supporters to a rally that ended in gunfire that killed seven people, officials said on Tuesday.

The assembly on Monday, marking the third anniversary of the death of iconic Palestinian leader Yasser Arafat, was the biggest held by President Mahmoud Abbas's Fatah group in Gaza since Hamas Islamists seized the territory by force in June.

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 took 400 Fatah members into custody in a series of raids and dozens more were ordered to report to police stations for questioning.

The rare Fatah show of strength in the Gaza Strip broke up in chaos after gunfire rang out. Seven civilians, including a 12-year-old boy, were killed and 80 people wounded, medical officials said.

Fatah officials accused Hamas forces of opening fire at the crowd. Hamas said its men had come under attack from Fatah gunmen and returned fire.

"Whoever plays with fire will burn his fingers," said Sami Abu Zuhri, a Hamas official.

MOURNING PERIOD

Abbas, preparing for a U.S.-hosted conference with Israel later this month on Palestinian statehood, has ruled out new dialogue with Hamas until the group relinquishes control of the Gaza Strip.

He declared three days of mourning for the Gaza dead and ordered flags to fly at half-mast in the occupied West Bank, where Fatah holds sway.

Abbas vowed in a statement to "continue the struggle and remain steadfast in the face of occupation and separatists until our people regain their unity and national independence and establish a Palestinian state with East Jerusalem as its capital".

In the West Bank city of Nablus, where hundreds of Palestinian security officers have deployed in what is planned as a first stage of a campaign to improve law and order, thousands gathered on Tuesday to honour the late Arafat.

"We will uproot the terrorist Hamas faction," Nablus Governor Jamal al-Mheisen said in a speech to the crowd on Abbas's behalf.

Giving no timeframe, Mheisen told the gathering: "Abbas will return to Gaza, which will be under his control." (Writing by Jeffrey Heller in Jerusalem; Editing by Sami Aboudi)