British Prime Minister David Cameron warned Iran of "serious consequences" after protesters in Tehran broke into the British embassy and a UK compound during an anti-British demonstration.

Protestors break windows of a building inside the compound of the British embassy in Tehran November 29, 2011.[Photo/China Daily via Agencies]

Cameron described the attacks as "outrageous and indefensible," BBC reported.

The attacks broke out following sanctions imposed on Iran over its nuclear program, leaving offices ransacked and flags burned.

The British Foreign Secretary William Hague Tuesday said that his country "holds the Iranian government responsible" for failing to protect its embassy from the storming protestors.

He blamed Iran for the "grave breach of the Vienna Convention which requires the protection of diplomats and diplomatic premises under all circumstances."

Hague had spoken to the Iranian foreign minister in the strongest terms and demanded immediate steps to ensure the safety of the staff and of both embassy compounds.

The United States and European Union also condemned the attacks.

The British Foreign Office has summoned the Iranian charge d'affaires in London and Cameron had held a meeting of the highest level security committee to discuss the matter.

British nationals were warned against all but essential travel to Iran and any Britons in the country were advised to remain indoors, according to Xinhua report.

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