Israel carried out dozens of airstrikes in Iran, hitting nuclear and military sites and killing several commanders and nuclear scientists.
Iran immediately responded by launching drones against Israel, and called the Israeli attack "a declaration of war".
International calls for restraint are multiplying, as fears grow the Middle East could be on the threshold of a broader conflict.
Here is what we know:
What was hit?
The attacks started in the early hours of this morning, during a day of rest and prayer in Iran.
Israel hit a key underground nuclear site in Natanz several times, Iranian state television said, reporting that most damage was at "surface level".
Other key nuclear sites at Fordow and Isfahan were not immediately struck, said the UN nuclear watchdog, the International Atomic Energy Agency, citing Iranian authorities.
Additional strikes were reported against sites in Iran's northwest.
The head of Iran's Revolutionary Guards, Hossein Salami, and the chief of staff of its armed forces, Mohammad Bagheri, were killed, with replacements swiftly named by supreme leader Ali Khamenei.
The Revolutionary Guards said that its aerospace commander, Amirali Hajizadeh, was also killed.
He was in charge of Iran's ballistic missile forces.
Iranian media said several nuclear scientists were killed.
Iran state media put the death toll from Israeli attacks at 18 killed though other Iranian media sources reported over 70 killed and hundreds injured.
State television said senior Khamenei adviser Ali Shamkhani was hurt in one of the strikes.
Israel launched a fresh round of strikes today as explosions were reported in Tehran and various locations across Iran.
The Israeli raids will "continue as many days as it takes", Israeli Prime Minister Benjamin Netanyahu said.
Iran's response
Iran launched some 100 drones in retaliation, according to the Israeli military, which said "most" were intercepted outside Israeli territory.
The bigger concern is Iran's sizeable arsenal of ballistic missiles.
Mr Khamenei warned Israel faces a "bitter and painful" fate over the attacks.
Iran's foreign minister called the Israeli attacks "a declaration of war" and urged UN Security Council action.
The newly appointed head of Iran's Islamic Revolutionary Guard Corps said the "gates of hell" will open on Israel in response to the attacks.
Iran had previously warned it would hit US military bases in the Middle East if conflict occurred. The United States pulled out non-essential personnel from several sites days ahead of the Israeli attack.
Internet restrictions were imposed across Iran, the country's communications ministry said, adding they would be lifted "once normalcy returns".
US involvement?
Mr Trump said Israel warned him of its raids ahead of time, but insisted the United States was not involved.
He warned Iran that the "next planned attacks" will be "even more brutal" and said Tehran should cut a deal to roll back its nuclear programme "before there is nothing left".
The US leader has repeatedly said he will not allow Iran to develop nuclear weapons.
Tehran has long denied seeking atomic bombs, but had been enriching uranium to a level very close to be able to make them.
The United States and Iran had been holding talks on the issue.
The next round, scheduled for Sunday in Oman, now looked to be cancelled.
Israel, Mr Trump said, has a huge military arsenal thanks to the United States and "they know how to use it".
Mr Trump's secretary of state, Marco Rubio, said the United States would protect its forces in the Middle East.
"Let me be clear: Iran should not target US interests or personnel," Mr Rubio said.
This afternoon on his Truth Social platform, Donald Trump urged Iran to make a deal "before there is nothing left".
Reactions
The attack, and likely Iranian response, is fueling international alarm.
Many capitals were urging restraint, fearing the consequences if the Israel-Iran conflict widened and drew in the United States, and if Middle East oil production and shipments were impacted.
Oil prices leapt dramatically, trading sharply up to around $75 (€65) a barrel.
The leaders of France, Germany and the UK were to hold a call to discuss the Israeli strikes, Berlin said.
Irish citizens have been advised not to travel to Israel in the wake of the attacks.
Several airlines cancelled flights servicing the region, including Emirates, Qatar Airways, Air France and Lufthansa, while Syria closed its airspace.
In Tehran, lines of motorists formed at service stations for fuel, residents stocked up on supplies, and protests were held against the Israeli airstrikes.
