The high-stakes move was compelled by Israel's growing exasperation over unrelenting rocket fire and followed the collapse of Egyptian-led efforts to broker a cease-fire. It also came after a failed attempt by 13 militants to infiltrate Israel earlier Thursday through a tunnel under the Gaza-Israel border, only to be stopped by an Israeli strike at the mouth of the tunnel.
The military said the first major Israeli ground offensive in Gaza in just over five years was open-ended and would be carried out on several fronts in the coastal strip.
"Our forces, 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," Israel's chief military spokesman Brig. Gen. Motti Almoz said.
He called on Gaza residents to evacuate areas where the military is operating, warning the "military is operating there with very great force."
Hamas spokesman Fawzi Barhoum said Israel "will pay dearly" for the assault. "Hamas is ready for a confrontation," he said.
An official in the Gaza security operations room reported heavy Israeli tank shelling along Gaza's border areas with Israel since 8 p.m. Thursday.
"All border areas are under fire. 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."
Residents said the northern Gaza town of Beit Lahiya came under heavy Israeli shelling. "There is the sound of tank shells all the time," said Jamal Abu Samra, 42, a farmer in the area. He said his wife, six children, four brothers and their families were huddling on the ground floor of the family home.
"We don't have power since the afternoon so we are listing to the (battery-operated) radio to hear the news," he said.
He said the Israeli military sent text messages to residents urging them to leave the area. Abu Samra said he and his relatives decided to stay because they felt nowhere in Gaza is safe. "It is better to stay home than move anywhere," he said.
Thousands of soldiers backed by tanks and huge DC9 bulldozers entered the Hamas-ruled territory after 10 p.m. Thursday, Israeli spokesman Lt. Col. Peter Lerner said.
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 a breaking point over the rockets.
Israel has faced pressure over mounting civilian casualties, especially after at least 13 Palestinian children under the age of 12 were killed by Israeli strikes in the past two days.
But Netanyahu may be sensing he has a degree of world legitimacy for action after Israel on Wednesday accepted a Egyptian cease-fire proposal which was essentially a return to the status quo ante - and Hamas then rejected it. Similarly, on Thursday, Hamas ended a "humanitarian lull" of several hours by immediately resuming rocket fire.
Thousands of Israeli soldiers had massed on the border with Gaza in recent days, waiting for the order to go in.
Israel had called up 48,000 reserve soldiers, and later Thursday the Cabinet authorized the military to call up 18,000 more, the military said.
The ground operation followed a brief truce earlier Thursday 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.
Netanyahu's office said he instructed the army to go ahead after Hamas rejected an Egyptian cease-fire plan earlier in the week and after Hamas militants tried to infiltrate into Israel through a tunnel from Gaza.
"In light of the despicable and relentless aggression by Hamas and the dangerous infiltration into Israel, Israel is obliged to protect its citizens," the statement said.
Israel last carried out a major ground offensive in Gaza in January 2009. During the three-week campaign, some 1,400 Palestinians were killed, including hundreds of civilians. Thirteen Israelis were killed as well. 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.
Deitch reported from Jerusalem. Associated Press writers Ibrahim Barzak in Gaza City and Yousur Alhlou in Jerusalem contributed to this report.