TEHRAN, Iran — Iranian protesters screaming "death to England" stormed the vast British Embassy compound and a diplomatic residence in Tehran on Tuesday, torched at least one vehicle, tore down the Union Jack, ransacked offices and briefly held six staff members captive during an officially approved protest of economic sanctions against Iran's suspect nuclear energy program.
The assault, reported by Iranian news services and broadcast on Iranian television, ended after several hours and constituted the most serious breach between Britain and Iran in more than 20 years. The images evoked memories of the siege of the U.S. Embassy after the Iranian revolution of 1979. The embassies are about a mile apart.
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 fire 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.
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 news agencies said about 50 protesters invaded the offices of the enormous walled compound housing the British Embassy and its manicured grounds, situated in a busy neighborhood in the heart of Tehran, while thousands of student protesters rallied outside, and that 200 to 300 others got into a British diplomatic residence facility a few miles north of the embassy, called Qolhak Garden. The residence facility also houses a school.
Iran's semiofficial Fars News Agency said police officers freed six British staff members who had been surrounded by the Qolhak Garden protesters and that 12 of those protesters were later arrested.
Britain's Foreign Secretary William Hague downplayed suggestions of a hostage situation, saying there had been "confusing" reports coming out of Iran.
He said that Britain held Iran's government responsible and promised "other, further and serious consequences."
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.
Iranian 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 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.
Information from the Associated Press and New York Times was used in this report.