Iran, through the semiofficial Fars News Agency, reports several people were injured, one critically, after police descended on protesters who stormed British Embassy grounds in central Tehran.
The protest, state media reports said, coincided with the first anniversary of the alleged assassination of an Iranian nuclear scientist by British intelligence officers.
British Foreign Secretary William Hague, in a statement, expressed his grave concern over the matter to his counterparts in Tehran. Iranian authorities, he said, are obligated under international law to ensure the protection of diplomats.
"The United Kingdom takes this irresponsible action extremely seriously," he stressed.
The Fars report described the protesters as "self-motivated." They managed to pry open the doors of the embassy and set British and Israeli flags on fire. Fars notes security officials arrested several of the demonstrators after gaining control of the situation.
Iran, state-funded broadcaster Press TV said, expressed regret over the incident. Hague, in his statement, confirmed the apology but stressed "this remains a very serious failure by the Iranian government."
Tehran noted that all diplomats at the British Embassy were safe.