Iran's Supreme Leader Ayatollah Ali Khamenei was killed, state media confirmed, after the United States and Israel launched a large-scale attack on Iran.
Iranian state television announced a 40-day mourning period and seven public holidays following the death of the Iranian leader, who had been in power since 1989.
"With the martyrdom of the supreme leader, his path and mission neither will be lost nor will be forgotten. On the other hand, they will be pursued with greater vigour and zeal," a presenter said.
US President Donald Trump has said that 48 Iranian leaders have been killed in the ongoing US-Israeli bombardments of the country.
"Nobody can believe the success we're having, 48 leaders are gone in one shot. And it's moving along rapidly," Mr Trump was quoted as saying in an interview by Fox News.
He said he would "be talking" to Iranian leaders but was vague on the timing and noted that much of the country's leadership was dead.
A senior Israeli official said earlier that the Iranian leader's body had been found after a strike, and Mr Trump said the United States worked closely with Israel to target the man who led Iran since 1989.
Some Iranians took to the streets cheering with joy, setting off fireworks and playing celebratory music after reports of Mr Khamenei's death emerged, according to witnesses and video footage.
The celebrations in Tehran began shortly after 11pm local time, according to multiple witnesses and audio recordings.
People were not, however, coming out en masse to celebrate, according to social media, as many Iranians remain fearful following the deadly crackdown on mass anti-government protests in January.
The thousands who did gather in the centre of Iran's capital were instead mourning Mr Khamenei's death.
The mourners, dressed mostly in black and some crying, chanted "death to America" and "death to Israel" in Enghelab (Revolution) Square, with many waving Iran's flags and holding photos of Mr Khamenei.
Watch: Thousands of mourners flood Tehran square after death of supreme leader
Alireza Arafi has been appointed as the jurist member of Iran's Leadership Council, a body tasked with fulfilling the supreme leader's role until the Assembly of Experts elects a new leader, ISNA news agency reported.
A cleric member of the Guardian Council, Mr Arafi will be part of the temporary Leadership Council alongside President Masoud Pezeshkian and Chief Justice Gholamhossein Mohseni Ejei.
Russian President Vladimir Putin expressed condolences to Iran's president over Ayatollah Khamenei "assassination", according to the Kremlin.
He called the killing a "cynical violation" of "morality and international law".
China said it "strongly condemns" the killing, and called again for a halt to military actions.
The killing was "a serious violation of Iran's sovereignty and security, a trampling on the aims and principles of the UN Charter and the basic norms of international relations", Beijing's foreign ministry said.
However, France can only "be satisfied" with Mr Khamenei's death, government spokeswoman Maud Bregeon said.
"Khamenei was a bloodthirsty dictator who oppressed his people, degraded women, young people and minorities, and was recently responsible for the deaths of thousands of civilians in his country and in the region. We can therefore only be satisfied with his demise," Ms Bregeon said to French broadcaster RTL.
Israel said it had launched another wave of strikes on Iran, as Iranians grappled with uncertainty after the killing of their supreme leader in the US and Israeli attacks that threaten to destabilise the wider Middle East.
The Israeli military said it had launched a broad wave of strikes in the heart of the Iranian capital, Tehran, while several explosions were heard in the city this morning.
US President Donald Trump said the air strikes aimed to end a decades-long threat from Iran and ensure it could not develop a nuclear weapon, as he sought to justify a risky gambit that seemed to contradict his professed opposition to American involvement in complex overseas conflicts.
"This is not only justice for the people of Iran, but for all great Americans, and those people from many countries throughout the world, that have been killed or mutilated by Khamenei and his gang of bloodthirsty thugs," Mr Trump wrote on Truth Social after Khamenei's body was found.
Experts said that while the deaths of Mr Khamenei and other Iranian leaders would deal the country a major blow, it would not necessarily spell the end of Iran's entrenched clerical rule or the Revolutionary Guards' sway over the population.
Watch: Eyewitness video shows Iranians toppling an Ayatollah monument in Galleh Dar, Fars Province
Iran's armed forces vowed in a statement that they would soon retaliate again with their biggest offensive against US bases and Israel.
Iran's security chief Ali Larijani vowed to hit the US and Israel with a force never experienced before.
"Yesterday Iran fired missiles at the United States and Israel, and they did hurt. Today we will hit them with a force that they have never experienced before," Mr Larijani said in a post on X.
President Trump, in a similar refrain, said the United States would hit Iran with "force that has never been seen before" if the country retaliated.
"Iran just stated that they are going to hit very hard today, harder than they have ever hit before," Mr Trump wrote on his Truth Social platform.
"THEY BETTER NOT DO THAT, HOWEVER, BECAUSE IF THEY DO, WE WILL HIT THEM WITH A FORCE THAT HAS NEVER BEEN SEEN BEFORE!"
Death toll at Minab School rises to 153 - report
The Red Crescent said strikes in Iran had left 201 people dead and injured hundreds more.
Iran's judiciary confirmed that Ali Shamkhani, a top adviser to Khamenei, and General Mohammad Pakpour, the head of Iran's powerful Revolutionary Guards, were among those killed.
Iranian news agency Mizan said today that the death toll from a strike on Minab School in Iran's south on Saturday had risen to 153.
Israel's military today that it was "not aware" of any US or Israeli strike on the school and that it was "operating in an extremely accurate manner".
Capt Tim Hawkins, a spokesperson for US Central Command, said the US was "aware of reports concerning civilian harm resulting from ongoing military operations. We take these reports seriously and are looking into them."
Fresh blasts were heard across the Gulf cities of Dubai, Doha, Abu and Manama this morning, after a day of Iranian strikes in retaliation for US and Israeli attacks, while at least nine people were killed in a missile attack in the Israeli city of Beit Shemesh
Air raid sirens repeatedly sounded across Israel, warning residents of an incoming attack, while in Tel Aviv, a series of explosions were heard as Israel's sophisticated air defence system sought to intercept the latest Iranian offensive.
In a video message on social media, Mr Trump said the aim of the military campaign, which the US Department of Defense named Operation Epic Fury, was "eliminating imminent threats from the Iranian regime."
The Iranian people should "take over" the governance of their country, Mr Trump said in the video. "It will be yours to take," he said. "This will probably be your only chance for generations."
UN Secretary-General Antonio Guterres called for an immediate cessation of hostilities.
Israeli military operations over the past two years had already killed some of Iran's senior military officials and severely weakened several of Tehran's once-feared proxy forces across the Middle East.
After Israel pounded Iran in a 12-day air war in June, joined by the United States, both warned they would strike again if Iran persisted with its nuclear and ballistic missile programs.
Read more:
Iran's Khamenei: ruthless revolutionary at apex of Islamic republic
US-Iran crisis: who are the main players?
The tense and troubled history of US-Iran relations
