(Adds Fatah calls for general strike, paragraph 9)

By Nidal al-Mughrabi

GAZA, Sept 7 (Reuters) - The Western-backed Palestinian government said clashes on Friday between its supporters and Hamas security forces in the Gaza Strip marked the start of an Intifada against the Islamists ruling the territory.

Wielding clubs, Hamas security men beat protesters, hurled stun grenades and fired in the air to disperse open-air prayers the rival Fatah faction held in the Gaza Strip in defiance of a ban on such gatherings.

Medical officials said 20 people, some with gunshot wounds, were treated in hospital.

"What we saw in Gaza today was the beginning of a third Intifada, against the Hamas occupation," Palestinian Information Minister Reyad al-Maliki told a news conference in the West Bank city of Ramallah. "We bless this uprising."

Palestinians launched what they describe as uprisings against Israeli occupation in 1987 and 2000. Hamas took over the Gaza Strip three months ago after routing Fatah forces in a brief civil war.

"They are chasing and beating and arresting us as if they were occupation soldiers," said one young Fatah supporter in Gaza's Maghazi refugee camp, likening Hamas forces to Israelis.

The street showdowns had been widely expected after Hamas said it would not allow Fatah to conduct "political prayers" outdoors on the Muslim rest day.

The Friday gatherings have become focal points for clashes between Hamas's Executive Force security wing that polices the territory, and members of Fatah, once the dominant faction in the Gaza Strip and now holding sway only in the West Bank.

Responding to the latest confrontations, a Fatah-led meeting of factions in the Palestine Liberation Organisation in Gaza called for a general strike in the territory on Sunday and demanded Hamas apologise for the actions of its security forces.

JOURNALISTS DETAINED

The Executive Force took away three Palestinian journalists -- two working for Japanese television and the third man a photographer for the Associated Press -- and roughed up five other reporters during Friday's protests, witnesses said.

The three detained journalists were later released, their employers said. The Jerusalem-based Foreign Press Association demanded an end to a "coordinated ... policy of harassment" of the media.

The Hamas administration in the Gaza Strip also seized three members of Fatah's local leadership and an adviser to West Bank-based President Mahmoud Abbas, saying they had instigated "chaos". The three Fatah men were released later in the day, said a spokesman for the Hamas-run Interior Ministry.

Speaking at a spot in Gaza City where Fatah members gathered for weekly worship, Youssef al-Zahar, a Hamas security officer, told reporters the movement was "trying to prevent people from using prayers to incite chaos and sabotage public property".

In Ramallah, Abbas said: "We are witnessing another new cycle of crimes committed by the putschist forces against our people in the Gaza Strip".

The Western-backed leader has described Hamas's routing of his Fatah forces in Gaza in fighting in June as a coup. Hamas has said Abbas acted unlawfully in subsequently dismissing a unity government it formed with Fatah in March. (Additional reporting by Mohammed Assadi in Ramallah)

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