Iran has launched hundreds of aerial drones and missiles at Israel, marking a widely anticipated reprisal attack.
It is the first time Iran has attacked Israel directly following a years-long shadow war between the two foes.
The Israeli military said Israel and other countries had intercepted more than 300 cruise missiles and drones, mostly outside Israeli airspace.
The barrage had caused very little damage, the Israeli military said. It warned people to remain alert.
US President Joe Biden said "we helped Israel take down nearly all" of missiles and drones as he expressed strong condemnation for the attack.
"Iran and its proxies operating out of Yemen, Syria and Iraq launched an unprecedented air attack against military facilities in Israel," he said.
Iran's Islamic Revolution Guard Corps (IRGC) said the attack was aimed at "specific targets".
Iran had vowed to retaliate for a strike on its consulate in Syria on 1 April which killed seven IRGC officers, including a top commander. It accused Israel of carrying out that attack, but Israel neither confirmed nor denied it.
Following the attack Israel's Prime Minister Benjamin Netanyahu vowed "together we will win" but it is unclear what Israel's response will be.
President Biden said he had reaffirmed "America's ironclad commitment to the security of Israel".
Israel Defense Forces (IDF) spokesman Rear Admiral Daniel Hagari said some Iranian missiles had hit inside Israel, causing minor damage to a military base but no casualties.
Israel's ambulance service said a seven-year-old Bedouin girl had been injured by shrapnel from falling debris in the southern Arad region.
Mr Hagari said the widescale attack was a "major escalation" and said Israel and allies had operated at full force to defend Israel.
In a separate briefing, he said Iran had fired more than 300 projectiles at Israel overnight, 99% of which were shot down. He added that some of the launches came from Iraq and Yemen.
Israel's Defence Minister Yoav Gallant said "very little damage was caused" but warned the "campaign is not over yet" and said Israel must "remain alert".
Two US officials told CBS, the BBC's US partner, that American forces had shot down several drones, but did not specify where or how they were intercepted.
The UK Ministry of Defence said RAF jets had been deployed in Iraq and Syria to intercept "any airborne attacks within range of our existing missions".
Sirens sounded across Israel and loud explosions were heard over Jerusalem, with air defence systems shooting down objects over the city.
Iran's IRGC - the most powerful branch of its armed forces - said it had launched the attack "in retaliation against the Zionist regime's [Israel] repeated crimes, including the attack on the Iranian embassy's consulate in Damascus".
President Biden cut short a trip to Delaware to return to the White House as tensions mounted on Saturday.
After speaking to Mr Netanyahu later he said he would convene "my fellow G7 leaders to co-ordinate a united diplomatic response to Iran's brazen attack".
UK Prime Minister Rishi Sunak condemned Iran's "reckless" attack, vowing that the UK would "continue to stand up for Israel's security and that of all our regional partners".
UN Secretary General António Guterres issued a statement saying he "strongly condemn[ed] the serious escalation represented by the large-scale attack launched on Israel" by Iran.
He said he was calling for "an immediate cessation of these hostilities" and for all sides to exercise maximum restraint.
"Neither the region nor the world can afford another war," he warned.
The UN Security Council will convene later for an emergency meeting over Iran's attack on Israel, its president Vanessa Frazier said.
Earlier this week, Israel's defence and foreign ministers warned that if Iran attacked Israel, Israel would strike back inside Iran.
Iranian armed forces chief of staff Major General Mohammad Bagheri told state television that Iran's response would be "much larger than tonight's military action if Israel retaliates against Iran", Reuters reported.
He added that the US had been warned not to back an Israeli response.
By Raffi Berg
Additional reporting: Laurence Peter, Emily Atkinson and Doug Faulkner
(BBC News)
