Iranian President Ebrahim Raisi, a hard-line protégé of the country's supreme leader who helped oversee the mass executions of thousands in 1988 earning himself the nickname 'The Butcher of Tehran', has died aged 63.

Elected president in 2021, he went on to lead the country as it enriched uranium near weapons-grade levels and launched a major drone-and-missile attack on Israel.

Raisi's sudden death, along with Iran's foreign minister and other officials in the helicopter crash Sunday in northwestern Iran, came as Iran struggles with internal dissent and its relations with the wider world.

A cleric first, Raisi once kissed the Quran, the Islamic holy book, before the UN and spoke more like a preacher than a statesman when addressing the world.

But he gained notoriety after he was appointed Deputy prosecutor of Tehran in 1985 before going on to serve on the prosecution committee which - under the orders of then-Supreme Leader Ayatollah Ruhollah Khomeini - sentenced thousands of political prisoners to death.