Iran has summoned the British ambassador in Tehran to complain about what it says was the illegal seizure of an Iranian oil tanker.
British Royal Marines helped the authorities in Gibraltar seize the ship because of evidence it was heading to Syria in breach of EU sanctions.
Spain's acting foreign minister said the seizure of the ship - Grace 1 - was at the US's request.
An Iranian foreign ministry spokesman called the actions a "form of piracy".
The UK Foreign Office dismissed that as "nonsense".
Gibraltar port and law enforcement agencies detained the super tanker and its cargo on Thursday morning with the help of the marines.
The BBC has been told a team of about 30 marines, from 42 Commando, were flown from the UK to Gibraltar to help, at the request of the Gibraltar government.
The first marines to board the Panama-flagged ship descended by rope from a helicopter, as others approached in speed boats. No shots were fired.
A defence source described it as a "relatively benign operation" without major incident.
However, Iran's foreign ministry spokesman Abbas Mousavi was later quoted as saying the UK's ambassador in Tehran, Robert Macaire, had been summoned over the "illegal seizure" of the tanker.
Source: BBC