Lyse Doucet: This is an extraordinary moment Iran has been preparing for

52 minutes agoLyse DoucetChief international correspondentWatch: How US-Israel strikes on Iran unfolded in 90 secondsThis is a defining moment for the Iran" class="entity-link entity-organization" data-entity-id="8614" data-entity-type="organization">Islamic Republic of Iran.There had been a swirl of reports about the fate of the supreme leader ever since Saturday morning, when it was clear that his residence had been targeted in the first wave of strikes.Satellite images showed significant damage to his compound.The first response from Iran was that he had been taken to a safe place.Then came news that the 86-year-old cleric was to speak on state television, but nothing materialised.By early evening, the Israeli Prime Minister Benjamin Netanyahu, in a televised address, announced that "there are many signs" that the Supreme leader "is no longer".A series of reports in Israeli and US media, quoting unnamed officials, spoke of convincing evidence he was dead.All the while, Iranian officials continued to deny it.But then, several hours after US President Donald Trump announced the news on his social media platform, an Iranian state TV presenter tearfully announced the passing of " the steadfast mountain of the Islamic guardianship" who "drank the sweet pure draught of martyrdom."A period of forty days of mourning was announced and as the second day of war dawned, pro-government events started emerging to grieve his passing.But videos had also swiftly emerged through the night of scenes of celebration in some cities across Iran, and explosions of joy in Iranian communities in many countries greeting the end of his hardline rule and expressing hope that it marked the end of the Islamic regime.These are fateful moments in the turbulent history of the Iran" class="entity-link entity-organization" data-entity-id="8614" data-entity-type="organization">Islamic Republic of Iran - but its most powerful clerics and commanders have been preparing for it.Minds were concentrated during the 12-day war last June. On the first night alone, in the first wave of attacks, Israel was able to assassinate nine nuclear scientists, and a number of security chiefs. And in the days that followed, more senior scientists and at least 30 leading commanders were killed. It was made clear that the Ayatollah could also be in their sights.It was reported then that Khamenei, who spent the war in his special bunker, was drawing up lists of security officials who could immediately step into place to avoid any vacuum in the top echelons.And even before last year's hostilities, it was reported that Khamenei had instructed the Assembly of Experts, the body of some 88 senior clerics tasked with choosing a supreme leader, to be ready for every eventuality. The New York Times wrote that he had chosen "three senior clerics" as possible replacements if he were to be assassinated.There has been speculation for many years about who could take his place, including his son Mojtaba.It is not just the supreme leader who was killed in this first day of air strikes and targeted attacks. Those who are still in place, or have had to step into more senior roles, will want to send a message to the world that they are still firmly in charge and the succession will be seamless.But the end of the Ayatollah's 36-year rule will be a jolt to his supporters, most of all his aides and allies in the elite Islamic Revolutionary Guards Corps (IRGC) tasked with defending him and the Islamic revolution at home and beyond. But the BBC has verified videos showing groups of people celebrating the reports of his demise on the streets of Tehran and Karaj.Deeply suspicious of the west, especially the United States, and hostile to Israel, Khamenei has ruled with a firm grip, suppressing calls for reform and repeated waves of protests. These last few years of direct military conflict with Israel and America, and growing calls for change from his own people, saw him confronted with his greatest challenges.During our time in Tehran earlier this month, Iran felt like a different country. The pain and anger over the security crackdown, the worst in its history, which killed many thousands of Iranians, was still raw.As Khamenei's time at the helm is brought to a sudden end, questions will turn to his successor and whether a change at the top could also signal a shift in direction for the 47-year-old Islamic republic. No matter who emerges, their overriding goal will remain the same - the survival of an order that keeps the clerics and its powerful security forces in power.A war which is far from over is already unfolding in unpredictable and perilous ways.