(Vahid Salemi ~ Associated Press)
The hours-long assault on the British Embassy and a residential complex for staff -- in chaotic scenes reminiscent of the seizing of the U.S. Embassy in 1979 -- could push already frayed diplomatic ties toward the breaking point.
Iran's parliament approved a bill Sunday to downgrade relations with Britain, one of America's closest allies with diplomatic envoys in the Islamic Republic.
Calling Tuesday's attack "outrageous and indefensible," British Prime Minister David Cameron said that Iran's failure to defend the embassy and its staff was a disgrace and would have "serious consequences."
He said all embassy staff had been accounted for and praised Britain's ambassador to Iran for handling a "dangerous situation with calm and professionalism."
Sorting out who to blame may be difficult.
The late-afternoon demonstration outside the British Embassy was organized by pro-government groups at universities and Islamic seminaries, and could not have taken place without official sanction. However, such anti-Western rallies often draw ultraconservative factions such as the basiji, a paramilitary group run by the powerful Revolutionary Guard that is directly controlled by Iran's ruling theocracy.
Riot police initially clashed with mobs in attempts to hold them back, but protesters surged past cordons and scaled the walls at the embassy complex, which they pelted with gasoline bombs and stones. Flames shot out of a sport utility vehicle parked outside the brick building and occupiers tossed papers apparently looted from an office.
"Death to England!" some cried outside the compound in the first significant assault of a foreign diplomatic area in Iran in years.
Inside the compound, protesters replaced the British flag with a banner in the name of 7th-century Shiite saint, Imam Hussein. One man showed a picture of Queen Elizabeth II apparently taken off a wall.
Chants called for the closure of the embassy and called it a "spy den" -- the same phrase used after militants stormed the U.S. Embassy in Tehran after the 1979 Islamic Revolution and held 52 hostages for 444 days. In the early moments of that siege, protesters tossed out papers from the compound and pulled down the U.S. flag. Washington and Tehran have had no diplomatic relations since then.
It was not immediately clear Tuesday if the protesters entered the main embassy building or only reached auxiliary sites.
In another part of Tehran, the official IRNA news agency said about 300 protesters entered a complex used for embassy staff and other officials and replaced British flags with Iranian ones.
Iranian media said six workers were detained by protesters at that site -- with some describing them as hostages -- but the semiofficial Fars news agency said they were released after negotiations with police. Their nationalities were not given.
Britain's Foreign Secretary William Hague downplayed suggestions of a hostage situation, saying there had been "confusing" reports coming out of Iran.
By nightfall -- more than three hours after the assaults began -- Iranian authorities appeared to have regained control of both British compounds. Riot police surrounded the embassy compound and officials said all protesters were driven out.
But sporadic clashes persisted, including some where police fired tear gas to disperse crowds, according to Fars. Some protesters were arrested, it said.
Iran's state TV said Iran's Foreign Ministry expressed regret about "unacceptable behavior" of protesters, saying Iran respects international agreements to protect diplomatic sites.
In Washington, President Barack Obama said he was "deeply disturbed" by Tuesday's events and urged the Iranian government to hold those responsible to account.
"For rioters to essentially be able to overrun the embassy and set it on fire is an indication that the Iranian government is not taking its international obligations seriously," Obama said.
Cameron also condemned Iran for "its unacceptable failure to protect diplomats in line with international law."
"The Iranian government must immediately ensure the continued safety of our staff, return all property and secure the compound immediately. Those responsible for this criminal attack must be prosecuted," he said.
The rally outside the British Embassy -- on a main street in downtown Tehran about a mile from the former U.S. Embassy -- included protesters carrying photographs of nuclear scientist Majid Shahriari, who was killed last year in an attack that Iran blamed on Israeli and British spy services.
The U.S. and many allies fear that Iran's nuclear program could eventually lead to nuclear weapons. Tehran says it only seeks reactors for energy and research, but will not give up the technology to make its own nuclear fuel.
On Monday, the U.S., Britain and Canada announced more sanctions intended to further isolate Iran's economy.
Tensions with Britain date back to the 19th century when the Persian monarchy gave huge industrial concessions to London, which later included significant control over Iran's oil industry. In 1953, Britain and the U.S. helped organized a coup that overthrew a nationalist prime minister and restored the pro-Western shah to power.
In recent years, Iran was angered by Britain's decision in 2007 to honor author Salman Rushdie with a knighthood. Rushdie went into hiding after Iran's Ayatollah Khomeini issued a 1989 fatwa, or religious edict, ordering Muslims to kill him because his novel "The Satanic Verses" allegedly insulted Islam.
In March 2007, Iran detained 15 British sailors and marines for allegedly entering the country's territorial waters in the Gulf -- a claim Britain denies. The 15 were released after nearly two weeks in captivity.
In 2006, angry mobs burned the Danish flag and attacked Danish and other Western embassies in Tehran to protest the reprinting of a cartoon deemed insulting of the Prophet Muhammad in the Nordic country's newspapers.
Murphy reported from Dubai, United Arab Emirates. Associated Press writer David Stringer in London contributed to this report.