On New Year's Day, Supreme Leader Ayatollah Ali Khamenei taunted Donald Trump on his favorite platform, Twitter, after the President vowed Iran would be held "fully responsible" for any lives lost at U.S. facilities abroad.

"You can't do anything," Khamenei boldly declared.

Less than 48 hours later, Gen. Qassem Soleimani, the head of the Islamic Revolutionary Guard Corps' elite Quds Force, was killed in an airstrike in Iraq ordered by "that guy" President Trump, with the ayatollah now claiming "harsh retaliation is waiting for the criminals whose filthy hands spilled his blood."

His words were echoed by Iran's president Hassan Rouhani, who promised to "take revenge for this heinous crime."

Rouhani also said Iran would "raise the flag" of Soleimani, the head of the Islamic Revolutionary Guard Corps' elite Quds Force, "in defense of the country's territorial integrity and the fight against terrorism and extremism in the region."

TRUMP ORDERS ATTACK THAT KILLS IRANIAN GEN. QASSIM SOLEIMANI, OTHER MILITARY OFFICIALS IN BAGHDAD, PENTAGON SAYS

The Pentagon confirmed Thursday evening that Trump had ordered the attack that killed Soleimani and other military officials at Baghdad International Airport. Iran’s top “shadow commander” was responsible for the deaths of hundreds of American and coalition service members and the wounding of thousands more, the State Department said.

The U.S. State Department issued a security alert Friday morning urging all U.S. citizens to “depart Iraq immediately” due to heightened tensions in the region.

“Due to Iranian-backed militia attacks at the U.S. Embassy compound, all consular operations are suspended. U.S. citizens should not approach the Embassy,” the alert said. The agency also shared phone numbers people concerned about U.S. citizens and other loved ones in Iraq could call toll free for information.

The State Department said the airstrike “was aimed at deterring future Iranian attack plans."

“General Soleimani was actively developing plans to attack American diplomats and service members in Iraq and throughout the region… The United States will continue to take all necessary action to protect our people and our interests wherever they are around the world.”

Brigadier General Esmail Qaani, who served as Soleimani’s deputy commander since 1997, will now take over the helm as head of the Quds Force, a move confirmed by Khamenei on Twitter later Friday.

Officials in New York City and Los Angeles have announced they’re taking precautionary measures in anticipation of a possible retaliatory attack. New York City Mayor Bill de Blasio said Thursday he’d spoken with top NYPD officials about immediate steps the department could take “to be vigilant against this threat for a long time to come.” LAPD said it was monitoring the situation in Iran but affirmed there has been no credible threat to the city.

Tens of thousands of people have taken to the streets of Tehran Friday to protest Soleimani's killing, chanting "Death to America" and holding up posters bearing his image, AFP reported. Similar demonstrations were held in a number of other Iranian cities, it added, citing state media.

U.S. Secretary of State Mike Pompeo also tweeted a video that he claimed showed Iraqis taking to the streets to celebrate the death of Soleimani. It was not immediately clear where or when the footage in the tweet was recorded.

TRUMP ORDERS ATTACK THAT KILLS IRANIAN GEN. QASSIM SOLEIMANI, OTHER MILITARY OFFICIALS IN BAGHDAD, PENTAGON SAYS

The Iranian foreign minister warned that the U.S. would bear all the consequences of the “foolish” military attack, claiming Soleimani's assassination would only escalate tensions in the region given that he was “THE most effective force” fighting terrorism carried out by the Islamic State.

Javad Zarif, the foreign minister of Islamic Republic of Iran, said on Twitter that “The US' act of international terrorism, targeting & assassinating General Soleimani—THE most effective force fighting Daesh (ISIS), Al Nusrah, Al Qaeda et al—is extremely dangerous & a foolish escalation.”

Many Democrats admitted that no Americans would mourn Soleimani's death but also raised concern that the escalation will put the U.S. on a crash course for a new conflict in the Mideast. House Speaker Nancy Pelosi released a statement saying that Trump ordered the airstrike “without the consultation of Congress.”

WASHINGTON POST BLASTED FOR REFERRING TO QASSIM SOLEIMANI AS IRAN'S 'MOST REVERED MILITARY LEADER'

“American leaders’ highest priority is to protect American lives and interests,” her statement said. “But we cannot put the lives of American service members, diplomats and others further at risk by engaging in provocative and disproportionate actions. Tonight’s airstrike risks provoking further dangerous escalation of violence.”

An adviser to Rouhani also quickly warned Trump of retaliation from Tehran.

"Trump through his gamble has dragged the U.S. into the most dangerous situation in the region," Hessameddin Ashena wrote on the social media app Telegram. "Whoever put his foot beyond the red line should be ready to face its consequences."

Soleimani is the military mastermind whom Pompeo had deemed equally as dangerous as Islamic State leader Abu Bakr al-Baghdadi. In October, Baghdadi killed himself during a U.S. raid on a compound in northwest Syria, seven months after the so-called ISIS "caliphate" crumbled as the terrorist group lost its final swath of Syrian territory in March.

The overnight attack occurred amid tensions with the U.S. after an Iran-backed militia attacked the U.S. Embassy in Baghdad, which was targeted Tuesday by angry mobs who were protesting recent U.S. airstrikes. The two-day siege outside of the U.S. Embassy in Baghdad came to an end Wednesday afternoon after dozens of pro-Iran militiamen and their supporters withdrew from the compound.

Soleimani was the long-running leader of the elite intelligence wing called Quds Force – which itself has been a designated terror group since 2007, and is estimated to be 20,000 strong. Considered one of the most powerful men in Iran, he routinely was referred to as its "shadow commander" or "spymaster."

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In April 2019, the State Department announced Iran was responsible for killing 608 U.S. troops during the Iraq War. Soleimani was the head of the Iranian and Iranian-backed forces carrying out those operations killing American troops. According to the State Department, 17 percent of all deaths of U.S. personnel in Iraq from 2003 to 2011 were orchestrated by Soleimani.

As recently as 2015, a travel ban and United Nations Security Council resolutions had barred Soleimani from leaving Iran.

Friday's Baghdad strike also killed Abu Mahdi al-Muhandis, the deputy commander of Iran-backed militias known as the Popular Mobilization Forces, a source told Fox News. In all, at least seven people were killed and at least three rockets were fired, officials told The Associated Press. An official with the Popular Mobilization Forces said its airport protocol officer, Mohammed Reda, also died.

Fox News’ Frank Miles, Lucas Tomlinson, John Roberts, Mike Arroyo and The Associated Press contributed to this report.