Britain said on Tuesday it was outraged at an incursion into its embassy compound in Tehran during which dozens of young Iranian men were reported to have entered buildings, thrown petrol bombs and burnt documents.
There has been a incursion by a significant number of demonstrators into our Embassy premises, including vandalism to our property, the Foreign Office said in a statement.
This is a fluid situation and details are still emerging, it added. We are outraged by this. It is utterly unacceptable and we condemn it.
The statement said that under international law Iran has a clear duty to protect diplomats and Embassies in their country.
We expect them to act urgently to bring the situation under control and ensure the safety of our staff and security of our property, it added.
Earlier, Foreign Secretary William Hague said he expected some other countries to follow Britain's lead in imposing financial sanctions on Iran and pledged robust action if Tehran reduces its diplomatic relations with London.
(Reporting by Stephen Addison; Editing by Mark Heinrich)