General Qassem Soleimani, the head of Iran's elite Islamic Revolutionary Guard Corps' (IRGC's) Quds Force, and architect of its regional security apparatus, has been killed following a US air raid at Baghdad's international airport on Friday.
Pre-dawn raid also killed Iraqi militia commander Abu Mahdi al-Muhandis in a major escalation of regional tensions.
Iran's Khamenei says 'vigorous revenge is waiting for the criminals' following Soleimani's death. Meanwhile, US Embassy in Baghdad urges American citizens to depart Iraq immediately - statement
The White House and the Pentagon confirmed the killing of Soleimani in Iraq, saying the attack was carried out at the direction of US President Donald Trump and was aimed at deterring future attacks allegedly being planned by Iran.
A three-day national mourning period has been declared in Iran in honour of Soleimani.
Iranian Leader Ayatollah Ali Khamenei paid tribute to him as a "martyr", and vowed a "vigorous revenge is waiting for the criminals."
Iranian President Hassan Rouhani echoed the Supreme Leader's threat of "revenge", while Foreign Minister Javad Zarif condemned the killing as an "act of international terrorism."
"The US bears responsibility for all consequences of its rogue adventurism," Zarif wrote on social media.
Iraqi officials and the state television reported that aside from Soleimani, Iraqi militia commander Abu Mahdi al-Muhandis was also killed in the pre-dawn raid.
Iran's IRGC as well as Iraq's Popular Mobilisation Forces (PMF) umbrella grouping of Iran-backed militias, also confirmed the deaths of Soleimani and al-Muhandis.
Killing Soleimani is equivalent to the Iranians "assassinating" the US defence secretary, or the commander of the US Central Command, she added.
"The president has taken this decision without debate in Congress, without any Congressional authority, it is probably an illegal act within the US domestic context."
In Tehran, Soleimani's death sent shockwaves among residents, who were awake when the news was announced, according to Al Jazeera's Dorsa Jabbari, who was reporting from the Iranian capital.
"With the news of his assassination, there is a tremendous amount of shock and anger that could follow, not only in Iran but across the Middle East," she said.
"His name is synonymous to Iranian national pride, no matter how he has been labelled outside of the country," Jabbari said, adding that hymns of mourning are being played on Iranian radio to mark Soleimani's death.
The area where the incident took place has been cordoned off, authorities told Al Jazeera, but the international airport remains in operation.
The incident took place near the base of the US-led coalition forces.
The attack occurred amid tensions with the US after an Iran-backed militia and its supporters breached the United States's Embassy in Baghdad.