Iran said its arch enemy the United States bore responsibility for the consequences after killing one of its top commanders, Qasem Soleimani, in an air strike Friday on Baghdad airport.
Iran's supreme leader Ayatollah Ali Khamenei warned of "severe revenge" for "the criminals who bloodied their foul hands with his blood", and vowed that "God willing, his work and his path will not be stopped".
The Revolutionary Guards confirmed the commander of its Quds Force had been killed by US forces in Baghdad, with Foreign Minister Mohammad Javad Zarif slamming the move as "foolish" and denouncing it as a "dangerous escalation".
"The Revolutionary Guards announces that the glorious commander of Islam, haj Qasem Soleimani, following a life of servitude, was martyred in an attack by America on Baghdad airport this morning," said a statement read out on state television.
The channel said the attack was carried out by US helicopters.
A black ribbon was displayed on the screen and a photo montage was shown with images of Soleimani smiling and praying, as Khamenei also declared three days of mourning.
Soleimani was one of the most popular figures in Iran and seen as a deadly adversary by America and its allies.
State television interrupted its regular programming to detail the commander's achievements in the region, pointing to many times he "foiled" US plans.
The Islamic republic's top security body called an emergency meeting over the attack.
"In a few hours, an extraordinary meeting of the Supreme National Security Council will be held to review the murderous attack on the vehicle of General Soleimani in Baghdad that lead to his martyrdom," ISNA quoted its secretariat spokesman Keyvan Khosravi as saying.
The foreign ministry also summoned an official from the Swiss embassy in Tehran, which represents US interests in Iran, to "strongly protest" the killing, according to a tweet by ministry spokesman Abbas Mousavi.
"The US' act of international terrorism, targeting and assassinating General Soleimani... is extremely dangerous and a foolish escalation," Zarif wrote on Twitter.
"The US bears responsibility for all consequences of its rogue adventurism," he added.
In a statement, the foreign ministry said the United States would pay for the "atrocity".
"The malice and stupidity of America's terrorist forces in assassinating General Soleimani, this hero martyr and the leader of combatting terrorism and extremism, will certainly further empower the tree of resistance in the region and the world," it said.
The ministry said it would "use all its political legal and international capacities to enact the decisions of the Supreme National Security Council to make the murderous and terrorist regime of America answer for this clear atrocity."
Zarif and his top deputies were in an urgent meeting at the ministry as of 07:00 AM (0330 GMT) to assess the situation, spokesman Abbas Mousavi said in a tweet.
A former head of the Guards said also revenge would be exacted on the United States for Soleimani's death.
"Soleimani joined his martyr brothers but we will exact terrible vengeance upon America," Mohsen Rezai, who currently heads the Expediency Council, wrote on Twitter.
Tehran-based analyst Mohammad Marandi said the US has "declared war" against Iran and Iraq, and "it's best for westerners to evacuate countries like UAE and Iraq immediately".
"By assassinating General Soleimani & Abu Mahdi al-Muhandis, both high ranking military officials, Trump has declared war against the armed forces of both Iran & Iraq. It is best for all American citizens to leave the region immediately. US occupiers will be forced to leave Iraq."
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)