As many as a million mourners lined the streets of Iran's capital on Monday as the coffin of a top Iranian general killed in a US airstrike in Baghdad on Friday passed through Tehran.
The targeted killing of Iranian Revolutionary Guard General Qassem Soleimani has already seen his replacement vow to take revenge, as European leaders urged restraint.
Tehran has already abandoned the remaining limits of its 2015 nuclear deal with world powers in response to the killing of Iran's second most powerful man, a development Number 10 described as "extremely concerning".
Foreign secretary Dominic Raab said he had spoken to his counterpart in Tehran on Monday, urging a diplomatic response as he distanced the UK from US President Donald Trump's suggestion that Iranian cultural sites could be targeted.
In joint statement on Monday, the Prime Minister, French President Emmanuel Macron and German Chancellor Angela Merkel said that while they were concerned by the “negative” role Iran has played in the region there was now “an urgent need for de-escalation”.
Mr Raab said the "overwhelming message" was the "importance of de-escalating the tensions and finding a diplomatic way through this crisis".
He said he would be talking further with European leaders as well as Middle Eastern partners and will be travelling to the US and Canada later this week.
But despite the calls to diffuse the situation, tensions remained high in Tehran, where Iran's Supreme Leader Ayatollah Ali Khamenei himself prayed over the caskets of Soleimani and others killed in the attack.
Khamenei, who had a close relationship with Soleimani, wept at one point during the traditional Muslim prayers for the dead.
Soleimani's daughter, Zeinab, directly threatened an attack on the US military in the Middle East while speaking to a crowd of hundreds of thousands in Tehran that stretched as far as the eye could see.
Iranian state TV put the crowd size at "millions", though that number could not be verified.
"The families of the American soldiers in western Asia ... will spend their days waiting for the death of their children," she said to cheers.
Soleimani's successor, Esmail Ghaani stood near Khamenei's side, as did Iranian President Hassan Rouhani and other top leaders in the Islamic Republic.
Ghaani vowed that "actions will be taken," in an interview with state TV.
Over the weekend the US President declared Iranian cultural sites as fair game for the US military, dismissing concerns within his own administration that doing so could constitute a war crime under international law, a statement the UK government have distanced themselves from pointing to laws preventing the destruction of cultural heritage.
"There are international conventions in place that prevent the destruction of cultural heritage," the Prime Minister's official spokesperson told a Westminster briefing.
Mr Raab reiterated the UK's response to Mr Trump's threat.
"We've been very clear that cultural sites are protected under international law.
"We expect that to be respected," the Foreign Secretary said on Monday.
- ITV News Correspondent John Ray in Beirut on the 'incendiary' situation in the Middle East that threatens to plunge the region into another conflict
Mr Trump also threatened to demand billions of dollars in compensation from Iraq or impose "sanctions like they've never seen before" if it goes through with expelling US troops.
On Sunday, the Iraqi voted in a favour of a bill requesting the withdrawal of US troops from the country.
The US has been criticised for apparently not giving warning of the attack to the UK, which has hundreds of troops deployed in Iraq.
In a statement from Downing Street on Monday, Number 10 insisted Britain's security partnership with the US remains "very close" despite Mr Trump not informing the UK of its plans to assassinate Soleimani.
The Prime Minister's official spokesperson said: "We have a very close security partnership with the United States, we are in regular dialogue at every level."
Asked if Mr Johnson was convinced the US drone strike was legal, the spokesperson said: "States have a right to take action such as this in self defence and the US have been clear that Soleimani was plotting imminent attacks on American diplomats and military personnel."
Iran insisted that it remains open to negotiations with European partners over its nuclear programme and it did not back off from earlier promises that it wouldn't seek a nuclear weapon.
Number 10 said it was in "everyone's interest that the deal remains in place,"
"It makes the world safer by taking the prospect of a nuclear-armed Iran firmly off the table," the PM's spokesperson said.
"We've always said the nuclear deal is a reciprocal deal and in light of Iran's announcement we are urgently speaking to partners about next steps."