"The arrest of our Ambassador in Tehran without grounds or explanation is a flagrant violation of international law," Foreign Secretary Dominic Raab said in a statement as he reacted to the arrest of Rob Macaire.

"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 deescalate tensions and engage in a diplomatic path forwards," British Foreign Secretary Dominic Raab said.

Rob Macaire was detained on Saturday night after attending a vigil for those who died when Iran's military shot down a passenger plane earlier this week, reported BBC.

He left when it turned into a protest before being arrested and accused of helping to organise the demonstrations, the British media further reported.

Meanwhile, Rob Macaire said he was not participating in any demonstration.

"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," he tweeted.

Protesters had taken to the streets in Iran's capital, Tehran, to vent anger at officials, calling them liars for having denied, then admitting, shooting down a Ukrainian passenger plane, reported BBC.

Meanwhile, Iran had summoned the UK ambassador, media reports said.

According to the Iranian Foreign Ministry Spokesman's office, Iran's official complaint has been communicated to Macaire and to the British government, reported IRNA news agency.

UK diplomat was reminded that presence of foreign ambassadors in illegal gatherings has nothing to do with their responsibilities as political representative of that country and is against 1961 Vienna Convention on Diplomatic Relations, reported the Iranian news agency.

Meanwhile,  Iranian Deputy Foreign Minister Abbas Araghchi said the UK Rob was arrested as an 'unknown foreigner' who was attending an illegal gathering.

Abbas Araghchi tweeted: "He wasn't detained, but arrested as unknown foreigner in an illegal gathering. When police informed me a man's arrested who claims to be UK Amb, I said IMPOSSIBLE! only after my phone conversation w him I identified, out of big surprise, that it's him. 15 min later he was free."

Amid these developments, US President Donald Trump tweeted: "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!"

Iran's military on Saturday said it shot down the Ukrainian passenger flight this week due to 'human error', reported Iran's state TV.

A Ukrainian plane which crashed earlier this week outside Tehran had flown close to a sensitive military site and brought down due to human error, General Staff of the Armed Forces of the Islamic Republic of Iran was quoted as saying by Press TV.At least 176 people were killed as Ukraine International Airlines flight PS752 crashed this week.

The incident occurred just hours after Iran carried out missile strikes on two airbases housing US forces in Iraq.