Our ambassador to Iran tweets:
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. Detained half an hour after leaving the area. Arresting diplomats is of course illegal, in all countries. See comments by Foreign Secretary.