Thousands of Iraqis have followed the coffin of Iranian military commander Qasem Soleimani, killed in a US airstrike at Baghdad airport on Friday.
Soleimani, viewed as a terrorist by the White House, was the architect of Iran's Middle East operations and Iran has vowed to take "severe revenge".
The funeral procession which began in Baghdad on Saturday marks the beginning of days of mourning for Soleimani.
The crowds in Baghdad also mourned the death of Abu Mahdi al-Muhandis, an Iraqi who commanded the Iranian-backed Kataib Hezbollah group and effectively led the Popular Mobilisation Forces (PMF) - an umbrella of militias in Iraq dominated by groups aligned with Iran.
Soleimani's body arrived Sunday in Iran where thousands of mourners thronged his coffin ahead of a grand funeral procession across the Islamic Republic amid soaring tensions between Iran and the US.
Iranian President Hassan Rouhani has promised members of the dead commander's family that Americans will "feel the impact" of their "criminal act... for years ahead".
The US drone strike killing Soleimani in Iraq Friday escalated the crisis between Tehran and Washington after months of trading attacks and threats that put the wider Middle East on edge. The conflict is rooted in Trump pulling out of Iran's atomic accord.
Iran has promised "harsh revenge" for the US attack, which shocked Iranians across all political lines. Many saw Soleimani as a pillar of the Islamic Republic at a moment when it is beset by US sanctions and recent anti-government protests.
Retaliation for Soleimani could potentially come through the proxy forces which he oversaw as the head of an elite unit within the paramilitary Revolutionary Guard. Soleimani's longtime deputy Esmail Ghaani already has taken over as the Quds Force's commander.
Late Saturday, a series of rockets launched in Baghdad fell inside or near the Green Zone, which houses government offices and foreign embassies, including the US Embassy.
Trump warned on Saturday that America was targeting 52 sites in Iran and would hit them "very fast and very hard" if the Islamic republic attacks American personnel or assets.
In a saber-rattling tweet, Trump said: "If they attack again, which I would strongly advise them not to do, we will hit them harder than they have ever been hit before!"
The 52 targets represented the 52 Americans held hostage in Iran after being seized at the US Embassy in 1979 during the country’s Islamic Revolution, Trump said.
The two countries have no diplomatic relations and on Sunday, Iran summoned the Swiss envoy representing US interests in Tehran to protest at “Trump’s hostile remarks”, according to Iranian state television.
Iran’s army chief, Major General Abdolrahim Mousavi, was quoted by state television on Sunday as saying the United States lacked the courage for military confrontation with Iran.
“In a potential conflict in the future, which I don’t think they (Americans) have the courage to carry out, there it will become clear where the numbers five and two will belong,” he said.
Trump said on Friday Soleimani had been plotting imminent attacks on US diplomats and military personnel. Democratic critics said the Republican president’s action was reckless and risked more bloodshed in a dangerous region.
Oman has called on the United States and Iran to seek dialogue to ease tensions, Oman News Agency reported on Sunday. Oman, which maintains friendly ties with both the United States and Iran, has previously been a go-between for the two countries.
British foreign minister Dominic Raab described Soleimani as a “regional menace” and said he was sympathetic to the situation the United States found itself in, while also calling for crisis diplomacy to avoid war.
“There is a route through which allows Iran to come in from out of the international cold,” Raab told Sky News. “We need to contain the nefarious actions of Iran but we also need to de-escalate and stabilize the situation.”
European Union foreign policy chief Josep Borrell urged Iran’s foreign minister by phone on Sunday to work to de-escalate the situation and invited him to Brussels to discuss ways of preserving world powers’ 2015 nuclear deal with Iran.
Trump’s withdrawal of the United States from the deal in 2018 and the reimposition of sanctions on Iran touched off a new spiral of tensions after a brief thaw following the accord.
Iran will decide on Sunday about its next step to further roll back its commitments to the nuclear deal, Foreign Ministry spokesman Abbas Mousavi was quoted as saying.
“Tonight, there will be a very important meeting to decide about our next nuclear step and the implementation of the deal ... considering the recent threats (by America) it should be underlined that in politics, all developments and threats are linked to each other,” state news agency IRNA quoted him as saying.
Iran previously has broken limits of its enrichment, its stockpiles, and its centrifuges, as well as restarted enrichment at an underground facility.
After the airstrike early Friday, the US-led coalition has scaled back operations and boosted "security and defensive measures" at bases hosting coalition forces in Iraq, a coalition official said on condition of anonymity according to regulations.
Meanwhile, the US has dispatched another 3,000 troops to Kuwait, the latest in a series of deployments in recent months as the standoff with Iran has worsened. Protesters held demonstrations in dozens of US cities Saturday over Trump's decisions to kill Soleimani and deploy more troops to the Mideast.
In a thinly veiled threat, one of the Iran-backed militias, Asaib Ahl al-Haq, or League of the Righteous, called on Iraqi security forces to stay at least a kilometer (0.6 miles) away from US bases starting Sunday night. However, US troops are invariably based in Iraqi military posts alongside local forces.
The Iranian parliament on Sunday opened with lawmakers in unison chanting: "Death to America!" Parliament speaker Ali Larijani compared Soleimani's killing to the 1953 CIA-backed coup that cemented the shah's power and to the US Navy's shootdown of an Iranian passenger plane in 1988 that killed 290 people. He also described American officials as following "the law of the jungle."
"Mr. Trump! This is the voice of Iranian nation. Listen!" Larijani said as lawmakers chanted.
A spokesman for Iran's armed forces, Gen. Abolfazl Shekarchi, likewise threatened the US by saying Iran and the "resistance front will decide the time, place and way" revenge will be carried out.
Iraq's parliament is meeting for an emergency session Sunday. Its government has come under mounting pressure to expel the 5,200 American troops who are based in the country to help prevent a resurgence of the ISIS group.
The US has ordered all citizens to leave Iraq and temporarily closed its embassy in Baghdad, where Iran-backed militiamen and their supporters staged two days of violent protests in which they breached the compound. Britain and France have warned their citizens to avoid or strictly limit travel in Iraq, as London said it would begin escorting ships through the Strait of Hormuz.
No one was hurt in the embassy protests, which came in response to US airstrikes that killed 25 Iran-backed militiamen in Iraq and Syria. The US blamed the militia for a rocket attack that killed a US contractor in northern Iraq.
