Video report by ITV News Senior International Correspondent John Irvine
A stampede at the funeral procession for the Iranian general killed in a US airstrike has left at least 56 people dead and 213 injured, state television reported.
It is estimated up to a million people have taken to the streets to pay tribute to General Qassem Soleimani in his hometown of Kerman, Iran.
Mourners dressed in black and carried posters bearing the image of Soleimani, a man whose slaying prompted Iran’s supreme leader to weep over his casket.
There is no information as to what set off the stampede.
Online videos showed people lying apparently lifeless, their faces covered by clothing.
Emergency crews performed CPR on others as people wailed in the background, crying out to God.
Pirhossein Koulivand, the head of Iran's emergency medical services, told state television: "Unfortunately as a result of the stampede, some of our compatriots have been injured and some have been killed during the funeral processions."
Authorities later delayed Soleimani's burial, citing concerns about the massive crowd that had gathered, the semi-official ISNA news agency said.
It did not say when the burial would take place.
Soleimani's death in a drone strike on Friday has sparked calls across Iran for revenge against America, drastically raising tensions across the Middle East.
But Secretary of State Mike Pompeo said the decision to kill Soleimani was the "right decision, we got it right."
Speaking to reporters, he said: "What we could see were continuing efforts on behalf of this terrorist to build out a network of campaign activities, that were going to potentially lead to the deaths of many more Americans."
The US blames Soleimani for the killing of American troops in Iraq and accused him of plotting new attacks just before his death.
In reaction to President Donald Trump's threat the US would illegally target Iranian cultural sites, Mr Pompeo said: "Every target that's being reviewed, every effort that's being made will be conducted inside the international rules of law."
He added: "At the end our policy is about protecting and defending the homeland and securing American lives, I know that the efforts we have taken, not only last week with the strike against Soleimani, but the strategy we have deployed has saved American lives."
Meanwhile Foreign Secretary Dominic Raab has travelled to Brussels to "de-escalate" the situation with his European counterparts, as he said the only winners of a war with Iran would be ISIS.
Speaking to reporters from RAF Northolt, he said: "What we are looking to do is to de-escalate the tensions with Iran.
"We are concerned that if we see a full-blown war, it would be very damaging and actually the terrorists, in particular Daesh (ISIS) would be the only winners."
Mr Raab spoke on Monday to his Iranian counterpart, Mohammed Javad Zarif, to stress the need for a diplomatic resolution to avoid a renewed conflict in the region.
As Iran mourns, Boris Johnson will be chairing a meeting of the National Security Council on Tuesday, to call on all sides in the Gulf crisis to draw back from all-out conflict.
MPs were updated on the latest situation in the region, on the first day back after the Christmas break.
Defence Secretary Ben Wallace has said the government has taken "urgent measures" to protect British personnel in Iran, including putting helicopters and ships in the region on standby.
He told the Commons: "General Soleimani was no friend or our allies in the region, he was not an advocate of a more peaceful or prosperous Middle East."
"Further conflict is in no one's interest, the only beneficiary would be the terrorists and extremists seeking to use the chaos as cover to advance their abhorrent objectives," Mr Wallace added.
The prime minister was not in the chamber for the statement and he has faced criticism he was slow to respond to the crisis – only returning to the UK at the weekend following his new year break on the private Caribbean island of Mustique.
Labour leader Jeremy Corbyn questioned why Mr Johnson was absent from the Commons and said the PM should be working towards restoring the Ian nuclear agreement.
He criticised the US administration, saying it is illegal to "assassinate an official of a foreign government in a third country" and said the attack had created an "incredibly dangerous time".
Hossein Salami, the leader of Iran's Revolutionary Guard, earlier in the day vowed to avenge Soleimani's death and threatened to "set ablaze" places supported by the US.
Salami made the pledge before a crowd of thousands gathered in a central square of Kerman.
He said as a martyr, Soleimani represented an even greater threat to Iran’s enemies, including longtime regional foe Israel.
“We will take revenge. We tell our enemies that we will retaliate but if they take another action we will set ablaze the places that they like and are passionate about," Salami said, drawing the cries of “death to Israel”.
His vow mirrored the demands of top Iranian officials - from Supreme Leader Ayatollah Ali Khamenei to others - as well as supporters across the Islamic Republic, demanding retaliation against America for a slaying that has drastically raised tensions across the Middle East.
The US has denied it will be withdrawing troops out of Iraq in the wake of Soleimani's killing, after a leaked letter appeared to suggest it was.
It comes after a vote by the Iraqi parliament over the weekend for the expulsion of US troops from the country.
The draft letter said troops would be "repositioning over the course of the coming days and weeks to prepare for onward movement."
The letter appeared to have been sent by Brigadier General William H Seely, head of the US military's task force in Iraq, to Abdul Amir, the deputy director of Combined Joint Operations.
But Defence Secretary Mark Esper told reporters the US is not pulling troops out of Iraq.
General Mark Milley, chairman of the Joint Chiefs of Staff, added that the "poorly written" letter was a draft which had been circulated internally by a US Marine commander and was an honest mistake that should never have got out.