Iran's supreme leader Ayatollah Khamenei wept in grief while thousands of mourners thronged Tehran's streets for the funeral of military commander Qasem Soleimani on Monday.
Revolutionary Guard General Soleimani was killed by a drone airstrike on the orders of US President Donald Trump on Friday.
The assassination of the architect of Iran's drive to extend its influence across the Middle East, has stoked concern around the globe that a broader regional conflict could erupt.
And as the coffins of Mr Soleimani and Iraqi militia leader Abu Mahdi al-Muhandis, who also died in Friday's attack in Baghdad, were passed over the heads of mourners, Mr Soleimani's successor vowed to expel US forces from the region in revenge.
Mr Trump has listed 52 Iranian targets, including cultural sites, that could be hit if Iran does retaliate with attacks on Americans or US assets, although US officials has since played down the president's reference to cultural targets.
General Esmail Ghaani, the new commander of the Quds Force, promised to "continue martyr Soleimani's cause as firmly as before".
Aerial footage from Tehran showed people packing thoroughfares and side streets and chanting "Death to America!", in a show of national unity.
Iran's Supreme Leader (C) leading a prayer over the caskets of slain military commander Qasem Soleimani and Iraqi paramilitary chief Abu Mahdi al-Muhandis. Source: AAP
Iran's demand that US forces quit the region gained traction on Sunday when neighbouring Iraq's parliament backed the prime minister's recommendation for foreign troops to be ordered out.
Iraq's rival Shi'ite leaders, including ones opposed to Iranian influence, have united since Friday's attack to call for the expulsion of US troops, who number about 5,000.
Prayers at Mr Soleimani's funeral in Tehran, which moves to the general's southern home city of Kerman on Tuesday, were led by Mr Khamenei, who wept as he spoke.
His daughter Zeinab Soleimani told mourners the US would face a "dark day" for her father's death.
Ismail Haniyeh, leader of the Palestinian militant group Hamas said "resistance against America" would continue.
Zeinab Soleimani, daughter of slain Iranian military commander Qasem Soleimani speaking during his funeral procession at Tehran University. Source: ABACA
Iran added to tensions on Sunday by dropping all limitations on its uranium enrichment - another step back from commitments under a landmark deal with major powers in 2015 to curtail Iran's nuclear programme that Trump abandoned in 2018.
The European Union has urged Iran to comply with the deal, with Commission President Ursula von der Leyen insisting on Monday: "We have to convince Iran that it's also in its own interest."
Mr Trump's adviser Kellyanne Conway President was still confident he could renegotiate a new nuclear agreement "if Iran wants to start behaving like a normal country".
Mr Trump stood by remarks that cultural sites were potential targets.
"They're allowed to kill our people. They're allowed to torture and maim our people. They're allowed to use roadside bombs and blow up our people. And we're not allowed to touch their cultural sites? It doesn't work that way," Mr Trump said.
Mr Trump also threatened sanctions against Iraq and said Baghdad would have to pay Washington for an airbase in Iraq if US troops were required to leave.
Britain said it understood why the US had acted but called for a de-escalation to avoid war and urged Iraq to allow UK troops to stay in the country to carry on with vital work.
