The Israeli army said "large numbers" of troops had moved into Gaza and the government called up thousands of reservists to join the ground offensive aimed at ending rocket and mortar attacks by Hamas and its militant allies.

Tanks opened up after crossing the border and casualties were reported around Gaza City and the northern towns of Beit Lahiya and Jabaliya.

Hamas forces fired back using mortars and rockets, witnesses said.

Gaza emergency services said one child was killed and 11 other children were wounded. They said two other Palestinians, including a woman, were killed east of Gaza City.

The Hamas television station said Israeli troops had Mbeen killed, but there was no immediate confirmation of this. Israel jammed Hamas the broadcasts, instead posting a caption reading: "Hamas leadership, your time is over."

In a statement, Prime Minister Ehud Olmert's office said the army would take over sites from which militants have fired rockets at Israel and deal a "hard blow" to the Islamists.

"Large numbers of forces are taking part in this stage of the operation including infantry, tanks, engineering forces, artillery and intelligence," a military spokesman said.

The military said a 20-nautical-mile naval blockade would be enforced along the coast.

Thousands of Israeli troops and tanks had encircled Gaza as artillery lobbed shells ahead of the offensive. F-16 jets roared over the enclave where at least 460 people have been killed in eight days of air attacks.

Militant rocket fire from Gaza into Israel on Saturday slightly wounded three people, and the exiled Hamas leader threatened a "black destiny" in the event of a ground invasion.

In one of the deadliest single strikes of the campaign so far, an Israeli jet hit a mosque in the northern town of Jabaliya, killing at least 16 people among more than 200 people at prayer, medics and witnesses said.

Four children -- two brothers aged 10 and 12 and two cousins under 16 -- were among the dead at the Ibrahim al-Maqadna mosque, medics said.

Dozens of wounded were dragged from the rubble.

Two Hamas military commanders were also killed in air strikes as pressure mounted on the Islamists. Their deaths came just two days after an Israeli missile killed top Hamas leader Nizar Rayyan.

Earlier a school in northern Gaza was destroyed in an air strike and a guard was killed. Israel said it had been used for firing rockets.

Israel has staged nearly 800 air raids against Hamas leaders and targets since "Operation Cast Lead" began on December 27.

Gaza medics say more than 460 Palestinians have been killed -- including 75 children -- and 2,310 wounded.

Four people have been killed in Israeli by more than 500 rockets fired from Gaza over the same period.

Exiled Hamas leader Khaled Meshaal warned Israel on Friday of a "black destiny" if it invaded. But Washington gave its key Middle East ally free rein to launch a ground operation.

In his weekly radio address, President George W. Bush called on Hamas "to turn away from terror" and rejected calls for a unilateral ceasefire that he said would allow the Islamists to continue hitting Israel with rockets.

There is mounting concern over the humanitarian impact of the Israeli operation. UN agencies have said there is a "critical" emergency in Gaza, where 80 percent of the 1.5 million population relies on international food aid.

Up to 100,000 Arab-Israelis attended a rally in Israel and tens of thousands joined demonstrations in London, Paris -- where police fought demonstrators -- and other European cities.

British Prime Minister Gordon Brown urged Olmert to halt the military action, Brown's office said.

"The prime minister has spoken again today to Prime Minister Olmert, and is pressing hard for an immediate ceasefire," a Downing Street statement said.

Palestinian president Mahmud Abbas was to meet French President counterpart Nicolas Sarkozy in Ramallah on Monday before travelling to New York to appeal for a ceasefire before the UN Security Council.

Jordan's King Abdullah II told Quartet Middle East peace envoy Tony Blair that the world's "silence" on the humanitarian crisis in Gaza was unacceptable.

Syrian President Bashar al-Assad on Saturday met Iran's Supreme National Security Council chief Saeed Jalili to discuss Gaza, state media reported. He also spoke with Russia's President Dmitry Medvedev.

Israel has kept Gaza virtually sealed off since Hamas seized power there in June 2007 from Fatah forces loyal to Abbas.

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