Britain has summoned the Iranian ambassador to the country to convey its strong objections to the brief arrest of Londons envoy to Tehran on Saturday, a spokesman for Prime Minister Boris Johnson said. "This was an unacceptable breach of the Vienna Convention and it needs to be investigated," the spokesman said on Monday. "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." British envoy Rob Macaire was arrested for several hours in front of Amir Kabir University after he took part in a vigil over the weekend for victims of a Ukraine jetliner that was mistakenly shot down by Iran’s Revolutionary Guards last week. Police briefly detained Macaire, who said he went to the vigil 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. Britain said its envoy was detained “without grounds or explanation” and in “flagrant violation of international law.” 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 didn’t believe them. But he said that once he spoke to Macaire by phone, he realized 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 organizing protests and called for his expulsion. Dozens of hard-liners later gathered outside the British Embassy, chanting “Death to England.” They also called for the ambassador to be expelled and the embassy to be closed.