"Martyr Soleimani is the international image of resistance and all lovers of resistance seek his revenge," he said in his message of condolence, in which he declared three days of national mourning, Leader.ir reported.
Soleimani, the head of IRGC's Quds Force, and Iraqi militia commander Abu Mahdi al-Muhandis were killed on Friday morning in the United States' airstrike on their convoy in Baghdad airport.
The Pentagon said in a statement that the attack was carried out on the order of US President Donald Trump with the aim of protecting American overseas personnel, Financial Tribune reported.
"At the direction of the president, the US military has taken decisive defensive action to protect US personnel abroad by killing Qasem Soleimani,” the statement read, according to Reuters.
"This strike was aimed at deterring future Iranian attack plans," it added.
The assassination was carried out following a rapid escalation between Tehran and Washington over militia activity in Iraq.
The US launched airstrikes on the Iran-allied Kataib Hezbollah militia bases on Sunday, accusing the group of killing an American civil contractor in a rocket attack on a coalition military base.
Iraqi militiamen stormed the US Embassy in Baghdad on Tuesday to protest against the US air raid in which about 25 fighters were killed.
The Pentagon claimed that Soleimani had "orchestrated" attacks on coalition bases in Iraq over the past few months and approved the "attacks" on the US Embassy. It also alleged that he was actively developing plans to attack Americans in Iraq and the Middle East.
Trump had said that Iran would be held accountable for lives lost or damage incurred in attacks on any US facilities, threatening that Iran will pay a very "big price".
Iran has denied involvement in the incidents, saying the protests stem from public hatred of the US and its interventionism in Iraq and other Middle East countries.
Ayatollah Khamenei said the path of resistance against the US and Israel will continue with strengthened motivation after Soleimani's martyrdom and "a definite victory awaits fighters in the holy war."
"The loss of our self-sacrificing and dear commander is bitter, but the continuation of the fight and achievement of final victory will leave the assassins and criminals more bitter," he said.
The Leader later appointed Quds Force Deputy Brigadier General Esmaeil Qa’ani as the commander of IRGC’s overseas operations arm.
Voices From Inside Iran
President Hassan Rouhani condemned the US violent act, describing it as an "evil and cowardly" measure and an indication of America's desperation in the region.
"Without a doubt, the great nation of Iran and other noble nations of the region will take revenge for this horrific crime," he said in his condolence note, President.ir reported.
The Supreme National Security Council and the top diplomats called an extraordinary meeting and the Swiss envoy representing the US interests in Iran was summoned to the Foreign Ministry, ISNA reported.
Foreign Minister Mohammad Javad Zarif stressed in a tweet that the US bears responsibility for all consequences of its "rogue adventurism".
"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," he said on his Twitter account, referring to terrorist groups that the Quds Force headed by Soleimani fought in the region, including the self-styled Islamic State terrorist group.
He also issued a statement, reassuring that the ministry will use all its political, legal and international resources to implement the decisions of the Supreme National Security Council and make the American administration answerable for its crimes.
IRGC Spokesman Lieutenant General Ramezan Sharif said the military force has opened a new chapter in its resistance agenda following the assassination of its commander.
"Certainly, the momentary happiness of America and the Zionists will not last long and will turn into grief," he said.
International Reaction
After the Pentagon confirmed the assassination of Soleimani, Trump posted a sole image of the US flag on his Twitter with no word.
Israel, the US closest ally, praised the United States and Trump for the strike on Friday.
“Trump is worthy of full appreciation for acting with determination, strongly and swiftly. We stand fully by the United States in its just battle for security, peace and self-defense,” Israeli Prime Minister Benjamin Netanyahu told reporters, the Times of Israel reported.
Iraqi Prime Minister Adel Abdul Mahdi, on the other hand, said the airstrike on Baghdad airport is an act of aggression on Iraq and a breach of its sovereignty that could lead to war in Iraq, the region and the world.
"The strike also violated the conditions of US military presence in Iraq and should be met with legislation that safeguards Iraq's security and sovereignty," he added.
There have been calls for restraint and de-escalation from both within the US and other nations.
Democratic Speaker of US House of Representatives Nancy Pelosi said that the attack "risks provoking further dangerous escalation of violence" that America and the world cannot afford.
She added that the strikes were conducted without an Authorization for Use of Military Force against Iran and without the consultation of Congress.
Former US vice president, Joe Biden, said in his statement that Trump had “just tossed a stick of dynamite into a tinderbox”.
“Iran will surely respond. We could be on the brink of a major conflict across the Middle East,” he said.
China appealed for restraint from all sides, especially the United States.
"China has always opposed the use of force in international relations," Chinese Foreign Ministry Spokesman Geng Shuang said at a daily press briefing, AFP reported.
"We urge the relevant sides, especially the United States, to remain calm and exercise restraint to avoid further escalating tensions."
He added that Iraq's sovereignty, independence and territorial integrity must be respected.
Russia warned that the US killing of top Iranian commander, who served the cause of protecting Iran's national interests with devotion, would boost tensions across the Middle East.
"The killing of Soleimani ... was an adventurist step that will increase tensions throughout the region," TASS quoted the foreign ministry as saying.
British Foreign Minister Dominic Raab blamed the Quds Force led by Soleimani for posing an “aggressive threat”, but urged all parties to deescalate following his passing.
"Further conflict is in none of our interests," he said.
Amelie de Montchalin, France's Europe minister, told French radio that president, Emmanuel Macron, would consult soon with players in the region.
"In such operations, when we can see an escalation is underway, what we want above all is stability and de-escalation," Montchalin said.
The French president has led mediatory efforts to defuse tensions between Iran and the US, which have reached a critical point following Washington's exit from the 2015 nuclear deal last year.
German Government Spokeswoman Ulrike Demmer also warned of rising tensions in the region.
"We stand before a dangerous escalation," she said, adding that Germany would work to deescalate the situation.