The shooting came two days after a rowdy protest outside the embassy over Britain's role in the occupation of Iraq. It also occurred amid tension between London and Tehran over Britain's arrest of a former Iranian ambassador who is wanted in connection with the 1994 bombing of a Jewish center in Argentina.
Five shots were fired from the street Wednesday, hitting offices on the first and second floors of the embassy but wounding nobody, the Foreign Office said.
The Associated Press saw what appeared to be three bullet holes in two embassy windows facing the street. About 30 Iranian police officers, some armed, were stationed outside the embassy after the incident. The pavement alongside the embassy wall was closed.
The official Islamic Republic News Agency reported witnesses as saying the shots were fired by gunmen on two motorbikes at 11:40 a.m. local time.
Calls to the embassy were not answered Wednesday afternoon.
British ambassador Richard Dalton was in contact with the Iranian Foreign Ministry, the Foreign Office said, adding it was "much too early" to say who was responsible for the shooting.
Iran's ambassador to Britain, Morteza Sarmadi, has returned to Tehran for consultations after talks with British Foreign Secretary Jack Straw over the Aug. 21 arrest of the former Iranian ambassador to Argentina, the Foreign Office said Wednesday.
Hade Soleimanpour, 47, has been charged in Argentina with conspiring to murder 85 people killed in the bombing of a Jewish community center in 1994.
Last week, a London court ordered Soleimanpour to remain in police custody until the beginning of his extradition case on Sept. 19. He was arrested in Durham, northern England, where he was studying.
Tehran has called for his release, saying the charges against him are politically motivated. But British officials say his arrest is a judicial matter and that they cannot interfere.
The Foreign Office said the Iranian ambassador's return to Tehran did not represent "a downgrading of relations."
About 100 people pelted the British Embassy in Tehran with stones on Monday. They were protesting what they called the "lack of security" in Iraq that contributed to Friday's assassination of Grand Ayatollah Mohammed Baqir al-Hakim in the city of Najaf, the official Islamic Republic News Agency reported. Police dispersed the protesters.
Britain is the chief ally of the United States in the occupation of Iraq.