Iran’s security forces have deployed in large numbers across the capital, expecting more protests after its Revolutionary Guard admitted to accidentally shooting down a passenger plane at a time of soaring tensions with the United States.

Riot police in black uniforms and helmets gathered in Vali-e Asr Square in the city as calls circulated for protests.

Revolutionary Guard members patrolled the city on motorbikes and plainclothes security men were also out in force.

The plane crash early on Wednesday killed all 176 people on board, mostly Iranians and Iranian-Canadians.

After initially blaming a technical failure, authorities finally admitted to accidentally shooting it down in the face of mounting evidence and accusations by western leaders.

The plane was shot down as Iran braced for retaliation after firing ballistic missiles at two bases in Iraq housing American forces.

People gather for a candlelight vigil in Tehran
People gather for a candlelight vigil in Tehran (Ebrahim Noroozi/AP/PA)

The ballistic missile attack, which caused no casualties, was a response to the killing of General Qassem Soleimani, Iran’s top general, in a US airstrike in Baghdad.

Iranians have expressed anger over the downing of the plane and the misleading explanations from senior officials in the wake of the tragedy.

Hundreds of students gathered at Tehran’s Shahid Beheshti University on Sunday to mourn the victims and protest against authorities for concealing the cause of the crash, the semi-official ISNA news agency reported. They later dispersed peacefully.

Others, including Foreign Minister Mohammad Javad Zarif, posted all-black photos on their Instagram accounts in mourning.

President Donald Trump, who has expressed support for past waves of anti-government demonstrations in Iran, addressed the country’s leaders in a tweet, saying “DO NOT KILL YOUR PROTESTERS.”

“The World is watching. More importantly, the USA is watching,” he tweeted.

Iranians took to the streets in November after the government hiked gas prices, holding large protests in several cities.

The government shut down internet access for days, making it difficult to gauge the scale of the protests and the subsequent crackdown. Amnesty International later said more than 300 people were killed.

A candlelight ceremony late on Saturday in Tehran turned into a protest, with hundreds of people chanting against the country’s leaders — including Supreme Leader Ayatollah Ali Khamenei — and police dispersing them with tear gas.

Police briefly detained the British ambassador to Iran, Rob Macaire, who says he went with the intention of attending the vigil and did not know it would turn into a protest.

“Can confirm I wasn’t taking part in any demonstrations!” he tweeted. “Went to an event advertised as a vigil for victims of #PS752 tragedy. Normal to want to pay respects — some of victims were British. I left after 5 mins, when some started chanting.”

He said he was arrested 30 minutes after leaving the area.

The UK said its envoy was detained “without grounds or explanation” and in “flagrant violation of international law”.

People gathered at Amri Kabir University in Tehran
People gathered at Amri Kabir University in Tehran (Ebrahim Noroozi/AP/PA)

“The Iranian government is at a cross-roads moment. It can continue its march towards pariah status with all the political and economic isolation that entails, or take steps to de-escalate tensions and engage in a diplomatic path forwards,” Foreign Secretary Dominic Raab said in a statement.

Iran’s deputy foreign minister Abbas Araghchi later tweeted that Mr Macaire was arrested “as an unknown foreigner in an illegal gathering”.

Mr Araghchi said when police informed him that a man was arrested who claimed to be the British ambassador he did not believe them.

But he said that once he spoke to Mr Macaire by phone he realised it was him, and that the ambassador was freed 15 minutes later.

Iran’s Foreign Ministry later summoned the British ambassador over his “illegal and inappropriate presence” at the protest, it said on its Telegram channel.

Alaeddin Boroujerdi, a member of Iran’s parliamentary committee on national security and foreign policy, accused the ambassador of organising protests and called for his expulsion.

Dozens of hard-liners later gathered outside the British Embassy, chanting “Death to England” and calling for the ambassador to be expelled and for the closure of the embassy. Police stood guard outside the facility.

Iranian media, meanwhile, focused on the admission of responsibility for the crash, with several newspapers calling for those responsible to apologize and resign.

The hardline daily Vatan-e Emrouz bore the front-page headline, “A sky full of sadness,” while the Hamshahri daily went with “Shame,” and the IRAN daily said “Unforgivable.”