The US and Israel launched an attack Saturday on Iran, with the first apparent strike happening near the offices of Supreme Leader Ayatollah Ali Khamenei.
Iranian media reported strikes nationwide, and smoke could be seen rising from the capital.
US President Donald Trump said in a video posted on social media that the US had begun "major combat operations in Iran".
LIVE UPDATES: US launches 'major combat operations in Iran'
Trump appealed to the Iranian people to "take over your government", saying "it will be yours to take".
He claimed Iran has continued to develop its nuclear program and plans to develop missiles to reach US.
The attack comes as the United States has assembled a vast fleet of fighter jets and warships in the region to try to pressure Iran into a deal over its nuclear program.
It wasn't immediately clear whether the 86-year-old Khamenei had been in his offices at the time. He hasn't been seen publicly in days as tensions with the United States have grown.
Roads to Khamenei's compound in downtown Tehran had been shut down by authorities as other blasts rang out across the capital.
Trump tells Iranian officials to lay down arms or 'face death'
Trump called the attacks on Iran "a noble mission," saying they were necessary because of Iran's pursuit of nuclear weapons and missile systems that could reach the US.
He called on Iranian officials to "lay down your arms" or "you will face certain death," and encouraged the Iranian people to "take over your government — it will be yours to take."
Despite claims that last year's US strike against Iran's nuclear facilities destroyed the country's nuclear program, Trump said that Iran "attempted to rebuild their nuclear program and to continue developing the long-range missiles that can now threaten our very good friends and allies in Europe, our troops stationed overseas, and could soon reach the American homeland".
Trump said that Saturday's strikes would be part of "a massive and ongoing operation" that will aim to "destroy their missiles and raze their missile industry to the ground" as well as "annihilate their navy," and "ensure that the region's terrorist proxies can no longer destabilise the region or the world and attack our forces."
Iran blocked inspectors from nuclear sites
Iran has said it hasn't enriched uranium since June, but it has blocked international inspectors from visiting the sites the United States bombed during the 12-day war then.
Satellite photos analysed by The Associated Press have shown new activity at two of those sites, suggesting Iran is trying to assess and potentially recover material there.
Removing 'threats', Israel says
Targets in the Israeli campaign included Iran's military, symbols of government and intelligence targets, according to an official briefed on the operation, who spoke on condition of anonymity to discuss non-public information on the attack.
Trump had wanted a deal to constrain Iran's nuclear program, and he sees an opportunity while the country is struggling at home with growing dissent following nationwide protests.
Iran had hoped to avert a war, but maintains it has the right to enrich uranium and does not want to discuss other issues, like its long-range missile program or support for armed groups like Hamas and Hezbollah.
It wasn't immediately clear if Iran would immediately strike back, but it had warned that American military personnel and bases spread across the region would be targets for any retaliation.
The Israeli government declared a state of emergency because of the expectation of Iranian retaliation with drones and ballistic missiles.
Israeli Defence Minister Israel Katz described the attack as being done "to remove threats." He did not immediately elaborate.
Several hospitals in Israel launched their emergency protocols, including moving patients and surgeries to underground facilities.
In Tehran, witnesses heard the first blast by Khamenei's office. Iranian state television later reported on the explosion, without offering a cause.
Sirens sounded across Israel at the same time. The Israeli military said that it had issued a "proactive alert to prepare the public for the possibility of missiles being launched toward the state of Israel."
More explosions struck Iran's capital after Israel said it was attacking the country. Authorities have offered no casualty information from the strikes.
Meanwhile, Iran shut down its airspace and mobile phone services were cut.
The warning to pilots came out as explosions rang out across Tehran, the Iranian capital.
Neighbouring Iraq closed its airspace, according to the Ministry of Transport.
Australia quick to react, issues urgent warning
Any Australians who are in Iran have been urged to leave the country, as the government issued a swift statement after news of the attacks broke on Saturday evening.
"There is a risk of reprisal attacks and further escalation across the region," it said in an updated travel advisory.
"Closure of airspace around transit hubs may impact flights globally, causing delays and cancellations."
It also warned the Australian Embassy in Tehran had suspended its operations, meaning the ability for it to offer support for stranded Australians is "extremely limited".
The Department of Foreign Affairs and Trade (DFAT) also urged those in Israel and Lebanon to consider their future plans, advising "the security situation could deteriorate rapidly throughout Israel with little or no notice".
"The Australian Government has directed the departure of all dependants of Australian officials posted to Israel in response to the deteriorating security situation in the Middle East," they said.
– Reported with Associated Press and CNN.
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