Hundreds of Iranian protesters have burned British flags outside the UK embassy and chanted: “Death to England”.
Some in the crowds held placards reading “Down with England” outside the British embassy in Tehran on Sunday following the brief arrest of the UK’s ambassador to Iran.
British and Israeli flags were set alight by protesters as tensions continue to escalate following the US assassination of Iranian general Qassem Soleimani earlier this month.
The British ambassador to Iran, Rob Macaire, was arrested after being caught up in a demonstration in Tehran’s Amir Kabir University.
Mr Macaire says he was attending a vigil for the 176 killed in a plane crash near Tehran last week.
Iran has admitted it shot down the Ukrainian passenger jet by mistake.
Iranian protesters called for the ambassador to be expelled and the embassy to be closed.
On Monday, Iran's ambassador to the UK was summoned by the Foreign Office in response to the "unacceptable" arrest of the British ambassador in Tehran, Downing Street said.
The prime minister's official spokesman said: "In relation to the arrest of the UK's ambassador over the weekend, I would say this was an unacceptable breach of the Vienna Convention and it needs to be investigated.
"We are seeking full assurances from the Iranian Government that this will never happen again.
"The FCO has summoned the Iranian ambassador today to convey our strong objections."
Videos posted online showed protesters shouting anti-government slogans and moving through rail stations and pavements in Tehran, many near Azadi, or Freedom, Square after an earlier call for people to demonstrate there.
Other videos suggested similar protests were taking place in other Iranian cities.
Meanwhile, in an emotional speech before parliament, the head of the Revolutionary Guard apologised for the shootdown 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 Gen Hossein Salami.
"I have never been this embarrassed in my entire life. Never."
Riot police in black uniforms and helmets earlier massed in Vali-e Asr Square, at Tehran University and other landmarks.
Revolutionary Guard members patrolled the city on motorbikes, and plainclothes security men were also out in force.
The plane crash early on Wednesday killed everyone on board, mostly Iranians and Iranian-Canadians.
Three British citizens were on board.
After initially pointing to a technical failure and insisting the armed forces were not to blame, authorities on Saturday admitted to accidentally shooting it down in the face of mounting evidence and accusations by Western leaders.
Iran downed the flight as it braced for possible American retaliation after firing ballistic missiles at two bases in Iraq housing US forces.
The missile attack, which caused no casualties, was a response to the killing of General Soleimani, Iran's top general, in a US air strike 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. They are also mourning the dead, which included many young people who were studying abroad.
To the leaders of Iran - DO NOT KILL YOUR PROTESTERS. Thousands have already been killed or imprisoned by you, and the World is watching. More importantly, the USA is watching. Turn your internet back on and let reporters roam free! Stop the killing of your great Iranian people!
— Donald J. Trump (@realDonaldTrump) January 12, 2020
US 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.
Thanks for the many goodwill messages. Can confirm I wasn’t taking part in any demonstrations! 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.
— Rob Macaire (@HMATehran) January 12, 2020
Mr Macaire said went to the vigil for the victims of the plane crash without knowing 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.