Video report by ITV News Political Correspondent Romilly Weeks
Iranian demonstrators defied a heavy police presence for a second day in a row after the country's leaders admitted to "unintentionally" shooting down a Ukrainian passenger plane, killing 176 people on board.
Videos posted online showed protesters shouting "shameless" and anti-government slogans as they gathered on streets in Tehran on Sunday, despite a heavy police presence.
Others videos suggested similar protests were taking place in other Iranian cities.
Protesters often wore hoods and covered their faces.
Some online videos purported to show police firing tear gas sporadically, although there was no immediate crackdown on demonstrators.
The plane was travelling to the Ukrainian capital, Kyiv, when it was shot down early on Wednesday near Tehran.
It was carrying mostly Iranians and Canadians, but Ukrainians, Swedes, Afghans and Brits were also on board, when it crashed.
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 came down shortly after Iran fired ballistic missiles at two bases in Iraq housing American forces.
The 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.
Meanwhile, Britain's ambassador in Iran was summoned to explain his "illegal and inappropriate presence" at an anti-government protest, the Iranian foreign ministry said.
It came as dozens of Iranian hard-liners were reported to have gathered outside the British embassy, chanting "Death to England" and calling for the ambassador to be expelled.
Rob Macaire was called in to the Ministry of Foreign Affairs after being arrested by police on Saturday as demonstrations broke out in Tehran over the downing of a Ukrainian airliner by Iranian forces.
Mr Macaire's summoning to the Ministry of Foreign Affairs threatened to further escalate the diplomatic row between London and Tehran after Foreign Secretary Dominic Raab denounced Mr Macaire's detention as a "flagrant violation" of international law.
It is illegal to arrest a diplomat.
The semi-official Tasnim news agency said he was one of a number of people arrested outside the Amir Kabir University on suspicion of organising, provoking and directing radical actions.
Mr Macaire denied he had been taking part in the demonstrations.
He said he been attending a vigil for the victims of Flight PS752, when protests broke out and he left.
"Can confirm I wasn't taking part in any demonstrations! Went to an event advertised as a vigil for victims of PS752 tragedy," he tweeted. "Normal to want to pay respects- some of victims were British. I left after five minutes, when some started chanting."
Mr Macaire said that he was arrested 30 minutes after leaving the area, although he was later released.
However, 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.
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."
He later tweeted the same message again in Farsi.
"The World is watching. More importantly, the USA is watching" he tweeted.
Meanwhile, in an emotional speech before parliament on Sunday, the head of the Revolutionary Guard apologised for the shooting down of the Ukrainian passenger plane and insisted it was a tragic mistake.
“I swear to almighty God that I wished I was on that plane and had crashed with them and burned but had not witnessed this tragic incident,” said General Hossein Salami.
“I have never been this embarrassed in my entire life. Never.”
After protests broke out on Saturday, criticising authorities for covering up the plane crash, riot police in black uniforms and helmets gathered in Vali-e Asr Square in the city as calls circulated for more demonstrations on Sunday.
Revolutionary Guard members also patrolled the city on motorbikes and plain clothes security men were also out in force.
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.
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.