Foreign Minister Israel Katz sent a letter of support to his British counterpart on Monday, after Iranian authorities arrested the UK’s ambassador.“I write to express my support and solidarity following the appalling behavior of the Iranian authorities towards your ambassador in Tehran,” he wrote to UK Foreign Secretary Dominic Raab. Katz called UK Ambassador to Iran Rob Macaire’s detention “undoubtedly a flagrant violation of the Vienna Convention, and as such was not just an assault on the United Kingdom but an attack upon the rules-based international system as a whole.”“This behavior by the Iranian regime deserves the unreserved condemnation by all responsible members of the international community,” he wrote. “Please know that Israel stands shoulder to shoulder with the UK.”Earlier Monday, Britain summoned the Iranian ambassador to the UK to convey its strong objections to Macaire’s brief arrest.“This was an unacceptable breach of the Vienna Convention, and it needs to be investigated,” a spokesman for UK Prime Minister Boris Johnson said. “We are seeking full assurances from the Iranian government that this will never happen again.”Iranian authorities accused Macaire of participating in an anti-regime protest.After his release, Macaire wrote on twitter: “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,” in which Iran shot down a Ukrainian airliner, killing 176.He said it was “normal to want to pay respects – some of victims were British. I left after 5 mins, when some started chanting.”Macaire recounted being “detained half an hour after leaving the area. Arresting diplomats is of course illegal, in all countries.”Reuters contributed to this report.