Offices were ransacked and flags burned in the attacks, which followed a demonstration against sanctions imposed on Iran over its nuclear programme.

The demonstrations followed a vote in Iran's parliament to reduce diplomatic ties with Britain in retaliation for imposing further sanctions.

Mr Cameron said the failure of the Iranian government to defend British staff and property was "a disgrace".

He said all British staff and their dependents had been accounted for and he praised Britain's ambassador to Iran, Dominick Chilcott, for handling a "dangerous situation with calm and professionalism".

US President Barack Obama said he was "deeply disturbed" by the attack.

"That kind of behaviour is not acceptable, and I strongly urge the Iranian government to hold those who are responsible to task," he said.

Germany, France and the EU also condemned the attack.

Hundreds of protesters - whom Iran described as "students" - had massed outside the embassy compound before scaling the walls and the gates.

A car was set alight, windows were broken, offices wrecked and paintings and other items dragged outside and dumped.

The occupations went on for several hours. By nightfall riot police had restored order and evicted the protesters.