Israeli ground troops and tanks cut swaths through the Gaza Strip early today, bisecting the coastal territory and surrounding its biggest city as the new phase of a devastating offensive against Hamas gained momentum.
Hamas in turn threatened to turn Gaza into a "graveyard" for Israeli forces.
"You entered like rats," Hamas spokesman Ismail Radwan told Israeli soldiers in a statement on Hamas' Al Aqsa TV. "Gaza will be a graveyard for you, God willing," he said.
Thousands of soldiers in three brigade-size formations pushed into Gaza after nightfall, beginning a long-awaited ground offensive after a week of intense aerial bombardment. Black smoke billowed over Gaza City at first light as bursts of machine gun fire rang out.
TV footage showed Israeli troops with night-vision goggles and camouflage face paint marching in single file. Artillery barrages preceded their advance, and they moved through fields and orchards following bomb-sniffing dogs to ensure their routes had not been booby-trapped.
The military said troops killed or wounded dozens of militant fighters, but Palestinian medical teams in Gaza, unable to move because of the fighting, could not provide casualty figures.
Hamas said only three of its fighters had been killed, and Gaza health officials said eight civilians also died, including a 12-year-old girl in the northern Gaza town of Beit Lahiya and four family members killed in an airstrike in southern Gaza.
Palestinian officials say nearly 480 people, including dozens of civilians, were killed in the week-long air offensive.
Army ambulances were seen bringing Israeli wounded to a hospital in the southern Israeli city of Beersheba. The military reported 30 Israeli troops were wounded, two seriously, in the opening hours of the offensive.
Defence Minister Ehud Barak predicted a long and difficult campaign in Gaza, a densely populated territory of 1.4 million where militants operate and easily hide among the crowded urban landscape.
The war will "not be short and it will not be easy", Mr Barak said in a nationally televised address late yesterday. "We do not seek war but we will not abandon our citizens to the ongoing Hamas attacks."
The ground operation is the second phase in an offensive which began as a week-long aerial onslaught aimed at halting Hamas rocket fire that has reached deeper and deeper into Israel, threatening major cities and one-eighth of Israel's population.
Rocket fire has persisted, however, and several rockets fell in Israel this morning, causing no casualties. In much of southern Israel school has been cancelled and life has been largely paralysed.
While the air offensive presented little risk for Israel's army, sending in ground troops is a much more dangerous proposition. Hamas is believed to have some 20,000 gunmen who know the dense urban landscape intimately. For months, Israeli leaders had resisted a ground invasion, fearing heavy casualties.
Israeli Prime Minister Ehud Olmert said he was aware of such risks, but decided that the government had no more choice.
"I want to be able to go to the Israeli public and all the mothers and say 'We did everything in a responsible manner'," Mr Olmert said in a statement released by his office. "In the end, we reached the moment where I had to decide to send out soldiers."
He stressed that the campaign's objective is to restore quiet to Israel's south, not to topple Hamas or reoccupy Gaza. Israel considers Hamas, which has controlled Gaza since June 2007 and is sworn to Israel's destruction, a terrorist group.
Israel has launched at least two other large ground offensives in Gaza since withdrawing its troops from the area in 2005. But the size of this latest operation dwarfs those previous attempts, with at least three times the firepower.
Israel has also called up tens of thousands of reserve soldiers, which defence officials said could enable a far broader ground offensive as the operation's third phase.
The troops could also be used in the event Palestinian militants in the West Bank or Hezbollah guerrillas in Lebanon decide to launch attacks. Hezbollah opened a war against Israel in 2006 when it was in the midst of a large operation in Gaza.
The officials spoke on condition of anonymity because the military's preparations are classified.
An armoured force south of Gaza City penetrated as deep as the abandoned settlement of Netzarim, which Israel left along with other Israeli communities when it pulled out of Gaza in 2005, both military officials and Palestinian witnesses said.
That move effectively cut off Gaza City, the territory's largest population centre, from the rest of Gaza to the south.
The offensive focused on northern Gaza, where most of the rockets are fired into Israel, but at least one incursion was reported in the southern part of the strip. Hamas uses smuggling tunnels along the southern border with Egypt to bring in weapons.
Warplanes struck about four dozen targets overnight, including tunnels, weapons storage facilities, areas used to launch mortars and squads of Hamas fighters, the military said.
Gunboats backed up the ground forces, attacking Hamas intelligence headquarters in Gaza City, rocket-launching areas and positions of Hamas marine forces.
Hamas was responding with mortar shells and rocket-propelled grenades. Field commanders communicated by walkie talkie, updating gunmen on the location of Israeli forces. Commanders told gunmen in the streets not to gather in big groups and not to use cell phones.
Hamas's TV and radio stations, broadcasting from secret locations after their offices were destroyed, remained on the air, broadcasting live coverage.
Ground forces had not entered major Gaza towns and cities by early this morning, instead fighting in rural communities and open areas militants often use to launch rockets and mortar rounds. Militants also fire from heavily populated neighbourhoods.
Residents of the small northern Gaza community of al-Attatra said soldiers moved from house to house by blowing holes through walls. Most of the houses were unoccupied, their residents already having fled.
Israel launched the air campaign against Gaza on December 27. Gaza health officials say more than 480 Palestinians were killed in the first eight days of the operation. The breakdown of combatants and civilians remains unclear, but the UN says at least 100 civilians were killed in the initial, aerial phase of the war.
Hundreds of rockets have hit Israel so far, and four Israelis have been killed.
The decision to send ground troops into Gaza was taken after Hamas kept up its rocket fire despite the aerial assault, government officials said. They spoke on condition of anonymity because discussions leading up to wartime decisions are confidential.
The ballooning death toll in Gaza - along with concerns of a looming humanitarian crisis - has aroused mounting world outrage, as evidenced by protests that drew tens of thousands of demonstrators in European capitals yesterday.
"There is a humanitarian crisis. It's impossible to say how many innocent women, innocent children and innocent babies are being caught up in this conflict, who are being maimed and killed," said Chris Gunness, a United Nations spokesman. "This offensive must stop."
Denunciations also came from the French government, which unsuccessfully proposed a two-day truce earlier this week, and from Egypt, which brokered the six-month truce whose breakdown preceded the Israeli offensive.
But the US put the blame squarely on Hamas. White House spokesman Gordon Johndroe said US officials have been in regular contact with the Israelis as well as officials from countries in the region and Europe.
"We continue to make clear to them our concerns for civilians, as well as the humanitarian situation," he said.
At an emergency consultation of the UN Security Council last night, the US blocked approval of a statement demanded by Arab countries calling for an immediate ceasefire and expressing serious concern at the escalation of violence.
Hamas began to emerge as Gaza's main power broker when it won Palestinian parliamentary elections three years ago. It has ruled the impoverished territory since seizing control from forces loyal to moderate Palestinian President Mahmoud Abbas in June 2007.
Palestinian medics and hospital doctors said 23 Palestinians had been killed in Israel's ground offensive since midnight.
They said three of the dead were Hamas fighters and the rest civilians.
Dr Said Judeh, of the Kamal Adwan Hospital in the northern Gaza town of Beit Lahiya, said eight people were killed in an Israeli artillery strike today. He said they were among a group of people trying to leave their homes for refuge in a nearby school. Beit Lahiya is the scene of some of the heaviest fighting.
Dr Judeh and Health Ministry official Moawiya Hassanain said five members of the same family in Beit Lahiya were killed in two separate artillery hits on their home.
Mr Hassanain also said a man and two teenage sons were killed in an Israeli airstrike in the southern town of Rafah.