The Israeli military has begun a ground offensive against Palestinian militants in the Gaza Strip, stepping up its 11-day-old military operation.
Troops and tanks were sent into Gaza to deal "a significant blow to Hamas", Israel said.
A Hamas spokesman said Israel would "pay a high price" for its actions.
Israel's Prime Minister Benjamin Netanyahu ordered the invasion after days of intensive rocket fire and air strikes between the two sides.
Gaza's health ministry said 24 Palestinians had been killed and 200 injured since the ground offensive began on Thursday night.
A five-month-old child was among the dead, Palestinian medics told AFP news agency. Israel said it had killed 14 "terrorists" overnight.
Israel meanwhile suffered its first military fatality, with the death of a soldier during the invasion, it said.
Some 258 Palestinians - three-quarters of them civilians - have died since the start of the wider Israeli operation on 8 July, officials in Gaza say.
An Israeli civilian was killed from mortar fire, and several Israelis have been seriously injured, Israeli medics say.
Announcing the ground offensive, the Israeli military said: "Following 10 days of Hamas attacks by land, air and sea, and after repeated rejections of offers to de-escalate the situation, the IDF [Israel Defense Forces] has initiated a ground operation within the Gaza Strip."
It said the goal was to "establish a reality in which Israeli residents can live in safety and security without continuous indiscriminate terror, while striking a significant blow to Hamas' terror infrastructure".
In Gaza City overnight, plumes of black smoke could be seen from the border area where Israeli troops were operating, AP news agency reported.
At the scene: Paul Adams, BBC News, Gaza City
It's hard to get a detailed picture, since the main focus of Israeli activity is some distance from downtown Gaza. But it's clear this is not yet a full-scale ground invasion.
Areas in the north and south of the Gaza Strip came under heavy bombardment overnight, but ground operations are so far limited. An Israeli military spokesman told us that troops had destroyed tunnels along the border.
He also confirmed that troops and armour remain just inside a largely unpopulated area in the north.
The Israeli military is testing the water, gauging the level of opposition before deciding whether or not to move deeper inside the Gaza Strip. The death of one of its own soldiers, at this early stage, will have come as a warning of the dangers ahead.
But Prime Minister Netanyahu seems determined to inflict greater damage on Hamas and that could mean sending his troops into densely populated areas. This will be dangerous for them, and potentially devastating for any Palestinian civilians in the way.
Israel said the initial phase was aimed at targeting tunnels Hamas has dug under the border with Israel to use in attacks.
On Wednesday 13 militants infiltrated into Israel through a tunnel aiming to attack a kibbutz, Israeli officials said. The Israeli military said it killed at least one of the militants, while the others retreated through the tunnel.
Reuters news agency said Palestinians reported heavy clashes along the length of the eastern border, as well as in the northern towns of Beit Hanoun and Beit Lahiya.
Military spokesman Gen Moti Almoz warned residents of Gaza to evacuate areas in which the army was operating. "This operation will be extended as much as necessary," he said.
Israel approved the drafting of 18,000 more reservists on Thursday evening, bringing the total of extra troops called up since 8 July to 65,000.
Hamas chief Khaled Meshaal said that the Israeli ground operation was "destined to failure".
"What the occupier Israel failed to achieve through its air and sea raids, it will not be able to achieve with a ground offensive", he said.
Palestinian Authority President Mahmoud Abbas said the ground operation would lead to "more bloodshed" and called on Israel to stop.
Mr Abbas, whose Fatah party recently agreed to a unity government with Hamas, was meeting Egyptian officials in Cairo amid efforts to negotiate a truce.
The BBC's Kevin Connolly in Jerusalem says it is possible that with the ground invasion Israel is seeking to improve its military position in advance of any deal, but that more probably it has concluded that a ceasefire at the moment in unlikely.
Israel says it has carried out more than 1,960 attacks on Gaza since 8 July, while militants have fired some 1,380 rockets at Israel. It says more than 50 rockets have been fired at Israel since the ground operation began.
The UN says at least 1,370 homes have been destroyed in Gaza and more than 18,000 people displaced in recent hostilities.
The ground offensive follows attempts in Cairo to negotiate a new ceasefire.
There was a temporary ceasefire on Thursday to allow Palestinians to stock up on supplies and aid workers to distribute water, food and hygiene kits.
The truce lasted for five hours, although both sides reported violations.