Pro-government gunmen have opened fire on demonstrators in Tehran, according to local reports, just hours after Trump warned 'do not kill your protesters.'

Local media footage showed a woman with blood pouring from a purported gunshot wound in the Iranian capital centre amid furious protests against the Ayatollah's regime.

Other images showed a 'militiaman' holding a shotgun running away from the scene as a crowd gathered around the woman who was allegedly killed not far from the iconic Azadi Square.

Thousands have flocked to the streets of the Iranian capital after the government admitted that a passenger jet carrying 176 people had been downed by a missile last week.

President Donald Trump had sent a message of solidarity to the demonstrators on Saturday, tweeting: 'There cannot be another massacre of peaceful protesters, nor an internet shutdown. The world is watching.'

Footage purports to show a 'militiaman' holding a shotgun running away from the scene as a crowd gathered around the woman who was allegedly killed

Blood spatters on the pavement not far from the iconic Azadi Square where a woman was said to have been shot dead by a pro-government gunman

It came after students packed the gates of Amirkabir University of Technology near the former US embassy to protest Iran's Supreme Leader Ayatollah Ali Khamenei.

On Sunday night, tensions in the city appeared to be turning to chaos with demonstrators setting fire to and tearing down posters of the Ayatollah and slain general Qassem Soleimani.

Amid shouts of 'Death to Khamenei' security forces opened fire with rubber bullets and tear gas, social media footage showed, as hundreds fled in terror.

Roads to Azadi Square were totally gridlocked by the demonstrations which continued late into the night.

Many reports claimed that those brutally breaking up the demonstrators were members of the Basij, a paramilitary volunteer wing of the Islamic Revolutionary Guard, and were acting on the orders of the regime.

Riot police armed with water cannon and batons were earlier seen at Amir Kabir, Sharif and Tehran universities as well as Enqelab Square.

Around 50 Basij militiamen brandishing paintball guns, potentially to mark protesters to authorities, were also seen near Amir Kabir.

Officers could be seen massing in Vali-e Asr Square in the city as calls circulated for protests.

A large black banner unveiled in the square bore the names of those killed in the plane crash.

Iranians gather to take part in an anti-government protest around Azadi square in Tehran Sunday night

Terrified masses could be seen fleeing the unfolding chaos on the streets of Tehran tonight as security forces were said to have opened fire with rubber bullets

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

Iran's top Guards commander briefed parliament on Sunday, a day after the armed forces said the Ukrainian airliner was shot down in error in an admission that sparked an angry demonstration.

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.

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

A demonstrator was seen tearing down a poster of the slain Iranian general Qassem Soleimani in the capital tonight

Roads to the iconic Azadi Square were totally gridlocked by the demonstrations on Sunday night

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

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.'

Iranians shout slogans against the government after a vigil held for the victims of the airplane of Ukrainian International Airlines that crashed near Imam Khomeini Airport turned into an anti-government protest outside Amirkabir University in Tehran on Saturday

People gather for a candlelight vigil to remember the victims of the Ukraine plane crash, at the gate of Amri Kabir University where some of the victims of the crash were former students

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'.

Revolutionary Guard Gen. Qassem Soleimani, the head of Iran's elite Quds Force, was killed in an airstrike at Baghdad's international airport

'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 Macaire was arrested 'as an unknown foreigner in an illegal gathering'.

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 toMacaire by phone he realized it was him, and that the ambassador was freed 15 minutes later.

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

One of the engines of the plane lies among the wreckage. The plane was shot down as Iran braced for retaliation after firing ballistic missiles at two bases in Iraq housing American forces

Officials stand near the wreckage after an Ukraine International Airlines Boeing 737-800 carrying 176 people crashed near Imam Khomeini Airport in Tehran, killing everyone on board

Dozens of hard-liners later gathered outside the British Embassy over the weekend, 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.'