The EU's diplomatic chief criticised Iran on Sunday for briefly detaining the British ambassador to Tehran, calling for "de-escalation".
Iranian authorities held ambassador Rob Macaire at a student protest to pay tribute to those killed when a Ukrainian airliner was shot down outside Tehran, killing 176 people including Britons.
The move triggered diplomatic protests from around the world, with London calling it a violation of international law and EU foreign policy chief Josep Borrell added his voice to the chorus of condemnation.
"Very concerned about the temporary detention of the UK Ambassador @HMATehran in Iran. Full respect of the Vienna convention is a must. The EU calls for de-escalation and space for diplomacy," Josep Borrell tweeted.
Macaire denied Iranian claims he was taking part in demonstrations, saying he went to an event to pay his respects to those killed in the crash and left after five minutes when chanting started.
Protests erupted in Iran on Saturday, with some students reportedly chanting "anti-regime" slogans amid concern about the authorities' handling of the disaster.
Iran's armed forces admitted on Saturday that the Ukraine International Airlines Boeing 737 was shot down, after denying for days Western claims it was brought down by a missile.