Hamas struck a defiant tone Thursday. A spokesman, Fawzi Barhoum, said Israel "will pay dearly" for the assault, its first major ground offensive in Gaza since January 2009. "Hamas is ready for a confrontation," he said.
Hamas has survived Israeli offensives in the past, including a major ground operation in January 2009 from which it emerged militarily weaker, but then recovered. Hamas has assembled thousands of rockets and built a system of underground bunkers.
The Israeli offensive began around 10 p.m. Thursday. The military said it was open-ended and would be carried out on several fronts in the coastal strip.
"Large ground forces accompanied by massive air force support, naval forces and intelligence, are taking over targets in Gaza, operating against tunnels and terror activists and infrastructure," said chief military spokesman Brig. Gen. Motti Almoz.
He called on Gaza residents to evacuate targeted areas, warning the "military is operating there with very great force."
As troops crossed into Gaza, the heavy thud of tank shells at intervals of just a few seconds could be heard across Gaza City.
An official in the Gaza security operations room said all of Gaza's border areas were being shelled, and that Hamas fighters were exchanging fire with Israeli troops near the northern Gaza town of Beit Hanoun.
"There is a tank shell every minute," said the official, speaking on condition of anonymity in line with briefing regulations. "There is also fire from the sea toward police checkpoints."
Gaza health officials said seven Palestinians were killed in the early stage of the ground operation, including a 3-month-old boy who died after a shell hit his family's Bedouin tent in southern Gaza. The body was evacuated on a donkey cart because ambulances couldn't reach the area due to heavy shelling, the officials said.
A resident of the northern town of Beit Lahiya, Jamal Abu Samra, said he was taking cover from the shelling by huddling on the ground floor of his home with his wife, six kids and two dozen other relatives.
"We don't have power since the afternoon so we are listing to the (battery-operated) radio to hear the news," he said.
Israeli government spokesman Mark Regev said the operation was focused on the tunnels dug by Hamas under the Gaza-Israel border. Earlier Thursday, 13 heavily armed Hamas militants had tried to sneak into Israel through such a tunnel, but were stopped by an airstrike at the mouth of the tunnel.
"For Israel to send ground forces into Gaza is not a light decision. Ultimately we understand the risks involved both for our own soldiers and the dangers of escalation," he said. "But we felt this was necessary ... to deal with this strategic threat posed by those tunnels, which can allow terrorists to infiltrate into Israel and cause mass death."
While the ultimate scale of Israel's ambition remained unclear, Prime Minister Benjamin Netanyahu had come under growing domestic pressure to ratchet up Israel's response to rocket fire that 10 days of airstrikes had failed to stem.
Israel has little stomach for the scale of casualties that a takeover of Gaza would likely entail, but Israeli public opinion appears to be nearly at a breaking point over the rockets.
Netanyahu may also have sensed he has a degree of international backing for action after Israel accepted an Egyptian cease-fire proposal Tuesday that was essentially a return to the status quo ante and Hamas then rejected it. Similarly, Hamas ended a "humanitarian lull" of several hours Thursday by immediately resuming rocket fire.
However, the ground offensive brought swift criticism from U.N. Secretary-General Ban Ki-moon, who said he regretted that despite his repeated urgings and "those of many regional and world leaders together, an already dangerous conflict has now escalated even further."
Both Ban and the Obama administration took Israel to task for the mounting civilian death toll in Gaza.
"I urge Israel to do far more to stop civilian casualties," said Ban. "There can be no military solution to this conflict."
Noting the deaths a day earlier of four boys who were killed on a Gaza beach by an Israeli strike, the State Department said the high civilian death toll in Gaza has been "heartbreaking."
Still, State Department spokeswoman Jen Psaki also criticized Hamas militants who continue to fire rockets and mortars into Israel, prolonging the latest round of violence.
Thousands of Israeli soldiers had massed on the border with Gaza in recent days, waiting for the order to go in.
Israel initially called up 48,000 reserve soldiers and later Thursday, the Cabinet authorized 18,000 more, the military said.
The ground operation followed a brief truce in which Israel held fire to allow Gazans to stock up on food and other necessities after being largely holed up at home since the conflict began last month.
Since July 8, Israeli strikes have hit more than 2,000 targets in Gaza and Hamas launched nearly 1,500 rockets at Israel, the Israeli military has said.
Israel last carried out a major ground offensive in Gaza in January 2009.
During that three-week campaign, some 1,400 Palestinians were killed, including hundreds of civilians. Thirteen Israelis also died. Israel has blamed Hamas for the heavy civilian toll, saying the militant group staged attacks from heavily populated residential areas, as well as mosques and schools.