What you need to know:
- Violence between Israel and the Palestinians has been surging since early last year.
Gun battles raged into the night after Palestinian militant group Hamas launched a surprise large-scale attack against Israel Saturday, firing thousands of rockets from Gaza and sending fighters to kill and abduct soldiers and civilians.
"We are at war," Israeli Prime Minister Benjamin Netanyahu told the stunned nation as the army retaliated by pounding targets in the blockaded enclave, where several residential tower blocks were reduced to rubble.
About 80 people were killed in Israel, medical services said, while Gaza authorities released a death toll of 232 in the conflict's bloodiest escalation in years which also left many hundreds wounded on both sides.
"The enemy will pay an unprecedented price," Netanyahu vowed after Hamas had launched its first such combined ground, air and sea offensive, half a century after the outbreak of the 1973 Arab-Israeli war.
As global concern grew and the UN Security Council called an emergency meeting for Sunday, President Joe Biden voiced "rock solid and unwavering" support for the US ally and warned "against any other party hostile to Israel seeking advantage in this situation".
As night fell, the Israeli army said its forces were still engaged in live gun battles in 22 Israel locations, in an ongoing operation labelled "Swords of Iron", as reservists were being called up.
"There are still 22 locations where we are engaging with terrorists that came into Israel, from the sea, from the land and from the air," said army spokesman Richard Hecht on what he labelled a "robust ground invasion".
Hamas earlier released images of several Israelis taken captive, and another army spokesman, Daniel Hagari, confirmed that "there are kidnapped soldiers and civilians.
"I can't give figures about them at the moment. It's a war crime committed by Hamas and they will pay the price."
Hecht said there was also a "severe hostage situation" in the Negev desert communities of Beeri and Ofakim east of Gaza.
Netanyahu vowed in another brief TV statement that the Israeli military would "use all its force to destroy Hamas's capabilities".
"We'll strike them to the bitter end and avenge with force this black day they brought on Israel and its people."
'So many bodies'
The Islamist group started the multi-pronged attack around 6:30 am (0330 GMT) with thousands of rockets aimed as far as Tel Aviv and Jerusalem, some bypassing the Iron Dome defence system and hitting buildings.
Hamas fighters -- travelling in ground vehicles, motorised paragliders and boats -- breached Gaza's security barrier and attacked nearby Israeli towns and military posts, opening fire on residents and passersby.
"Send help, please!" one Israeli woman sheltering with her two-year-old child pleaded as militants outside opened fire and tried to break into their safe room, Israeli media reported.
Bodies were seen lying on the streets of the Israeli town of Sderot near Gaza and inside cars, the windscreens shattered by hails of bullets.
"I saw many bodies, of terrorists and civilians," one man told AFP, standing beside covered corpses on a road near Gevim Kibbutz in southern Israel.
"So many bodies, so many bodies."
AFP journalists witnessed Palestinian armed men gather around a burning Israeli tank, and others driving a seized Israeli military Humvee back into Gaza, where they were met by cheering crowds.
'Gates of hell'
Israeli army Major General Ghasan Alyan warned that Hamas had "opened the gates of hell".
An AFP journalist in Gaza saw smoke billowing from the remains of a bombed residential tower which Gaza's interior ministry said contained 100 apartments.
Israel's military said it had warned residents to evacuate before targeting the multi-storey buildings used by Hamas.
The aid group Doctors without Borders said one strike had hit the enclave's Indonesian hospital and an ambulance outside Nasser Hospital in southern Gaza, causing multiple deaths.
Israel's state-run electricity company cut the power supply to Gaza as army flares lit up the night sky.
The escalation follows months of rising violence, mostly in the occupied West Bank, and tensions around Gaza's border and at contested holy sites in Jerusalem.
Before Saturday, at least 247 Palestinians, 32 Israelis and two foreigners had been killed this year, including combatants and civilians, according to Israeli and Palestinian officials.
Hamas labelled its attack "Operation Al-Aqsa Flood" and called on "resistance fighters in the West Bank" as well as in "Arab and Islamic nations" to join the battle.
"We decided to put an end to all the crimes of the occupation (Israel)," said its armed wing, the Ezzedine al-Qassam Brigades, claiming to have fired more than 5,000 rockets.
Hecht said Israel had counted more than 3,000 incoming rockets though the day.
Hamas chief Ismail Haniyeh claimed the group was on the "verge of a great victory".
"The cycle of intifadas (uprisings) and revolutions in the battle to liberate our land and our prisoners languishing in occupation prisons must be completed."
Air raid sirens wailed across southern and central Israel, as well as in Jerusalem, where incoming rockets were intercepted by air defence systems.
In Tel Aviv, a gaping hole was ripped into a building, with residents boarding a bus to flee to safety, and rockets also hit Ashkelon, north of Gaza.
The conflict sparked major disruption at Tel Aviv airport, where many carriers cancelled flights.
Schools will remain closed on Sunday, the start of the week in Israel.
Hamas took control of Gaza in 2007, leading to Israel's crippling blockade of the impoverished enclave of 2.3 million people.
Israel and Hamas have since fought several wars. The last major military exchange, in May, killed 34 Palestinians and one Israeli.
In northern Gaza on Saturday, hundreds of people fled their homes, carrying food and blankets, an AFP correspondent said.
Violence also erupted across the West Bank, including annexed east Jerusalem, with five Palestinians killed and 120 wounded in clashes with Israeli forces and settlers, Palestinian medical services said.
Western capitals condemned the wave of attacks by Hamas, which Israel, the United States and European Union consider a terrorist group.
European Commission President Ursula von der Leyen called the attack "terrorism in its most despicable form".
But Hamas drew support from other foes of Israel, with Iran's supreme leader declaring he was "proud" and Lebanese militant group Hezbollah praising the "heroic operation".
UN Middle East peace envoy Tor Wennesland warned of "a dangerous precipice" and called on all sides to "pull back from the brink".