From a CBS/AP report:
The students clashed with anti-riot police and chanted "the Embassy of Britain should be taken over" and "death to England." . . .
Iran's state television network earlier showed video of dozens of protesters in the embassy compound, at one point burning a presumably-looted portrait of the Queen and a British flag. Many dozens more people, including many women clad in black veils as mandated by the Islamic nation's laws, gathered outside the embassy compound's fence.
Via Twitter, the British foreign office has responded: "We are outraged by this. It is utterly unacceptable and we condemn it."
The Iranian state-run channel PressTV calls the attack a "rally outside [the] British embassy in Tehran."
UPDATE: Here's the full statement from the British foreign office:
“There has been a incursion by a significant number of demonstrators into our Embassy premises, including vandalism to our property.
“This is a fluid situation and details are still emerging.
“We are outraged by this. It is utterly unacceptable and we condemn it.
“Under international law , including the Vienna Convention, the Iranian Government have a clear duty to protect diplomats and Embassies in their country and expect them to act urgently to bring the situation under control and ensure the safety of our staff and security of our property."