Hundreds of thousands of mourners have turned out in Iran to receive home the remains of Qasem Soleimani, the general killed by a US drone strike in Iraq.
In Ahvaz, where Soleimani served during the Iran-Iraq war (1980-88), mourners chanted: "Death to America."
Soleimani was the architect of Iran's sphere of influence across the Middle East, and was considered to be the country's second most powerful man.
His assassination marked a significant escalation between Iran and the US.
Iran's Supreme Leader Ayatollah Khamenei, who had a close personal relationship with Soleimani, has warned of "severe revenge" for the attack, and analysts say Iran may pursue cyber-attacks against the US or traditional attacks on US targets or interests in the Middle East.
US President Donald Trump, who authorised the attack on Soleimani on Friday - an option refused by both Presidents George W Bush and Barack Obama as too risky - said on Saturday that the US was ready to strike 52 sites "important to Iran & the Iranian culture".
The United States just spent Two Trillion Dollars on Military Equipment. We are the biggest and by far the BEST in the World! If Iran attacks an American Base, or any American, we will be sending some of that brand new beautiful equipment their way...and without hesitation!— Donald J. Trump (@realDonaldTrump) January 5, 2020
Iran's Foreign Minister Mohammad Javad Zarif responded on Twitter, saying the killing of Soleimani was a breach of international law and that any targeting of cultural sites would constitute a war crime.
What is going on in Iran?
Thousands of black-clad mourners gathered early on Sunday morning in the streets in Ahvaz in the south-west of Iran, where Soleimani's body had arrived before dawn.
The channel showed crowds gathered in the city's Mollavi Square, waving flags and holding aloft portraits of Soleimani, who is seen by many in Iran as a hero because of his role in the Iran-Iraq war and his closeness to the supreme leader.
Similar scenes were later repeated in the city of Mashhad - the latest stage of three days of ceremonies throughout the country.
In the capital Tehran, members of parliament chanted "Death to America" for a few minutes during a session of the house, the Isna news agency reported.
"Trump, this is the voice of the Iranian nation, listen," Speaker Ali Larijani was quoted as saying.
Soleimani's body was flown back to Iran from Iraq alongside the bodies of five other Iranians killed in the drone strike, and the body of Abu Mahdi al-Muhandis, an Iraqi who had commanded the Iranian-backed Kataib Hezbollah group and had also been killed.
The bodies of Soleimani, al-Muhandis and the other victims were due to be flown to Tehran later on Sunday for more funeral events.
On Monday, the supreme leader is expected to pray over Soleimani's remains at Tehran University, followed by a procession through the city. The general's remains will then be taken to the holy city of Qom for a ceremony ahead of a funeral in his hometown of Kerman on Tuesday.
While there was mourning in Iran over Soleimani's death, there were also celebrations in the streets in Iraq and in Syria, where the general was instrumental in helping President Bashar al-Assad crush an uprising.
What impact is the killing having on Iraq?
Iraq finds itself in a difficult position as an ally both of neighbouring Iran and of the US. Thousands of US troops remain in the country to assist in the broader struggle against the Sunni Muslim Islamic State (IS).
A variety of Shia militia groups in Iraq are supported by Iran, and there are concerns that those parts of Iraq's Shia population sympathetic to Iran may have been alienated, and that militants may seek revenge for both the death of Soleimani and that of the Kataib Hezbollah leader.
Meanwhile, Iraq's foreign ministry has summoned the US ambassador over the drone strike, saying it considers it a "blatant violation of [Iraq's] sovereignty and breach of the agreement with the US-led coalition".
"The foreign ministry considers these US illegal military operations as an aggression," it added.
Prime Minister Adel Abdul Mahdi has been speaking to a special session of the Iraqi parliament where MPs are expected to debate a demand for the withdrawal of foreign troops from the country.
Kataib Hezbollah has warned Iraqi security forces to "stay clear of American bases by a distance not less [than] 1,000m [0.6 miles] starting Sunday evening", al-Mayadeen TV reports.
The US has advised its citizens to leave Iraq immediately and has deployed 3,000 additional troops to the Middle East.
What criticism does Trump face at home?
Speaking after the attack on Friday, President Trump told the world that he had taken the decision to assassinate Soleimani because the general had been "plotting imminent and sinister attacks" on US diplomats and military personnel in Iraq and elsewhere in the region.
The Trump administration decided not to notify Congressional leaders in advance of the attack, as previous presidents often did, and President Obama did before Osama Bin Laden was killed in Pakistan in 2011.
The White House sent its formal notification to Congress instead on Saturday, within the 48 hours after an attack that is required by US law.
Nancy Pelosi, the Democratic speaker of the House of Representatives, said in a statement that the White House notification raised "serious and urgent questions about the timing, manner and justification of the administration's decision to engage in hostilities against Iran".
Ms Pelosi said the decision to classify the entire document "suggests that the Congress and the American people are being left in the dark about our national security".
How has the attack strained relations between the US and its allies?
The surprise strike appeared to strain relations between the US and some European powers. French President Emmanuel Macron telephoned Iraq's acting prime minister to express support for the country's sovereignty. Iraq's leadership was not notified by the US ahead of the strike.
UK Prime Minister Boris Johnson had not by Sunday morning made any comment about the strike or elected to cut short his holiday on the Caribbean island of Mustique.
US Secretary of State Mike Pompeo criticised the European response to the attack.
Speaking to Fox News, he said: "Frankly, the Europeans haven't been as helpful as I wish that they could be. The Brits, the French, the Germans all need to understand that what we did, what the Americans did, saved lives in Europe as well."