Protesters have broken into the UK embassy compound in Iran's capital, Tehran, and vandalized embassy property during an anti-British demonstration, the BBC reported.
Militant students were shown on Iranian state TV smashing embassy windows with rocks, and shouting "death to England," according to the BBC. There were also reports that students were ransacking offices within the compound, and that the British flag was taken down, burnt, and replaced with the Iranian flag.
According to the AP, a Foreign Office source said it was checking on the well-being of workers and diplomats.
The semiofficial Mehr news agency said embassy staff had left the compound before the mobs entered, but it also said those who occupied the area had taken six staff as hostages. It did not give their nationalities and the report was later removed from the website without elaboration.
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The protest followed the UK's decision to impose further financial sanctions on Iran over its nuclear program.
Iran's parliament on Sunday voted to downgrade diplomatic relations with the UK in response to the sanctions, the BBC said.
The UK's Foreign Office said it was "outraged" by the actions of protesters at its embassy, and called for the Iranian government to control the situation.
"We are outraged by this. It is utterly unacceptable and we condemn it," the foreign office said in a statement.
"Under international law, including the Vienna Convention, the Iranian Government have a clear duty to protect diplomats and embassies in their country and we expect them to act urgently to bring the situation under control and ensure the safety of our staff and security of our property."
According to the BBC, an unconfirmed report from the Irna news agency claimed that a separate group of protesters had broken into another British embassy compound north of the city, and had seized classified documents.
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According to the Guardian, tensions between Britain and Iran date back to the 19th century, when the PErsian monarch gave industrial concessions to London, which included significant control over Iran's oil. However, relations between the two countries have become increasingly strained with allegations of Iran's nuclear programs by the west.
France also condemned the attack, the BBC reported.
"France expresses its full solidarity with the UK," French Foreign Minister Alain Juppe said.