GAZA CITY, Gaza Strip - Israeli tanks and infantry pushed into Gaza after nightfall Saturday, launching a ground offensive in a widening war that Israel's defence minister said would be neither short nor easy.

The ground operation was preceded by several hours of heavy artillery fire after dark, igniting targets that sent flames bursting into the night sky.

Machine-gun fire rattled as bright tracer rounds flashed through the darkness and the crash of hundreds of shells sent up streaks of fire.

Israeli artillery also fired illuminating rounds, sending streaks of bright light drifting down over Gaza's densely packed neighbourhoods. Gunbattles could be heard as troops crossed the border into Gaza, marching single file. They were backed by helicopter gunships and tanks.

Israeli security officials said the objective was not to reoccupy Gaza, but that the depth and intensity of the invasion would depend on parallel diplomatic efforts.

Israel's campaign "won't be easy and it won't be short," Defence Minister Ehud Barak said in a televised address shortly after the ground invasion began. "We do not seek war but we will not abandon our citizens to the ongoing Hamas (rocket) attacks.

Maj. Avital Leibovich, an Israeli army spokeswoman, told CNN: "We have many, many targets. To my estimation, it will be a lengthy operation."

Heavy Israeli artillery fire hit east of Gaza City in locations were Hamas fighters were deployed before the ground incursion began. The artillery shells were apparently intended to detonate Hamas explosive devices and mines planted along the border area before troops marched in.

Hamas has long prepared for Israel's invasion, digging tunnels and rigging some areas with explosives.

A text message sent by Hamas' military wing, Izzedine al-Qassam, said "the Zionists started approaching the trap which our fighters prepared for them."

Hamas said it also broadcast a Hebrew message on Israeli military radio frequencies promising to kill and kidnap the Israeli soldiers.

Hamas spokesman Ismail Radwan said in a televised speech that Gaza will "become a graveyard" for Israeli soldiers. He appeared on Hamas' Al Aqsa TV shortly after the ground offensive began but it was not immediately clear whether the appearance was live or taped.

The Israeli government said tens of thousands of reserve soldiers are being mobilized as the offensive in Gaza widens.

Artillery units joined Israel's Gaza offensive for the first time Saturday while warplanes and gunboats pounded more than 40 Hamas targets as well as a mosque, where 10 people were killed.

Air strikes waned during the day but gathered pace after dark.

"We will do all that is necessary to provide a different reality for southern Israel, which has been under constant attacks for the past eight years," Foreign Minister Tzipi Livni told Channel 2 TV.

The Israeli offensive has sparked large protests around the world over the past few days. Tens of thousands rallied Saturday in about a dozen European countries against the Israeli action.

Some hurled shoes at iron gates near the British prime minister's residence in London, in an echo of the Iraq journalist who angrily threw his shoes at President George W. Bush while he was visiting Iraq last month.

Tens of thousands of Israeli Arabs demonstrated in the northern town of Sakhnin, by far the biggest protest in Israel so far. Marchers held Palestinian flags and a smattering of green Hamas flags. But there were no reports of violence.

Protests involving hundreds of demonstrators also took place in Ottawa and Toronto, with additional rallies scheduled for Vancouver and Montreal.

Israel launched the offensive Dec. 27 in response to intensifying rocket fire by militants in Gaza.

Prior to the ground invasion, the operation had already killed well over 400 Palestinians, with upwards of 1,700 wounded. UN officials estimate that at least 25 per cent of the casualties have been civilians and that a serious humanitarian crisis has developed.

Israel, which has suffered four deaths from Hamas rocket fire, denies there is a humanitarian crisis in Gaza.

The use of artillery fire raised the possibility of higher civilian casualties. Artillery fire is less accurate than the precision-guided bombs and missiles used by the Israeli air force.

An artillery shell hit a house in Beit Lahiya after nightfall Saturday, wounding many people, according to members of the family living there. Ambulances could not immediately reach them because of the resulting fire, they said.

One air strike hit a mosque in the northern Gaza town of Beit Lahiya, killing 10 people and wounding 33, seven critically, according to a Palestinian health official.

The army also struck the American International School, the most prestigious educational institution in Gaza. The school is not connected to the U.S. government, but it teaches an American curriculum in English.

The air strike demolished the school's main building and killed a night watchman. Two other Palestinians were killed in a separate air strike, while four others, including a mid-level militant commander, died of wounds sustained earlier, Gaza health officials said.

Maxwell Gaylard, UN humanitarian co-ordinator for the Palestinians Territories, has estimated that a quarter of the Palestinians killed so far have been civilians and a "significant number" of the dead were women and children. He said some 2,000 people have been wounded.

"There is a critical emergency right now in the Gaza Strip," he said.