Iranian students stormed two British diplomatic sites in Tehran on Tuesday, vandalizing buildings and sparking international outrage in the process.

The students barged past police into the British Embassy in downtown Tehran and reportedly besieged a compound in the north of the city that houses British diplomatic staff, smashing windows, looting and setting at least one vehicle on fire.

British Prime Minister David Cameron called the events "outrageous and indefensible" and warned that the Iranian government would face "serious consequences" for failing to protect diplomats in line with international law.

Canada also condemned the attack on the U.K. embassy, with Foreign Affairs Minister John Baird calling on Iranian officials to fulfil their international obligations to protect all diplomatic staff in Iran.

"Iranian authorities failed to uphold their serious responsibilities under the Vienna Convention on Diplomatic Relations, namely, to safeguard diplomatic premises and ensure the safety of internationally protected persons," he said in a statement. "My officials have summoned the Iranian chargé d’affaires to Canada to convey our displeasure directly."

He added all Canadian diplomatic staff and facilities in Tehran are secure.

At the White House, U.S. President Barack Obama said he was "deeply disturbed" by the attacks and said it was a sign "that the Iranian government is not taking its international obligations seriously."

The French and Russian governments also condemned the attacks.

Flag replaced

The protesters broke into the embassy after clashing with anti-riot police, The Associated Press reported. "Death to England," some cried in the first significant assault of a foreign diplomatic area in Iran in years.

Smoke rose from some areas of the embassy grounds and the British flag was replaced with a banner in the name of seventh-century Shiite saint, Imam Hussein. Occupiers also tore down a picture of the Queen.

Iranian protesters break the windows of a British Embassy building, in Tehran, Iran, Tuesday, Nov. 29, 2011. (Vahid Salemi/The Associated Press)

The occupiers called for the closure of the embassy calling it a "spy den" — the same phrase used after militants stormed the U.S. Embassy in Tehran in 1979 and held 52 hostages for 444 days. In the early moments of the siege, protesters tossed out papers from the compound and hauled down the U.S. flag. Washington and Tehran have had no diplomatic relations since then.

The rally outside the British Embassy — on a main street in downtown Tehran — 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.

Outside the embassy, students from some universities and seminaries burned British flags and clashed with police.

Students support hard line leaders

Freelance journalist Thomas Erdbrink said from Tehran that staff may have vacated the embassy before the demonstration, which had been previously announced. But it was unclear whether diplomats remained at the compound in northern Tehran.

Britain's Foreign Office said it was in contact with embassy officials, and had been checking on the well-being of workers and diplomats, a Foreign Office spokeswoman said on condition of anonymity.

It also warned its citizens in Iran to "stay inside and keep a low profile."

Tensions between Iran and 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.

Protesters burn a British flag taken down from the British embassy in Tehran on Nov. 29, 2011. (Raheb Homavandi/Reuters)

But they have become increasingly strained as the West accuses Iran of trying to develop nuclear weapons — a charge Tehran denies.

Last week, the United Kingdom, Canada and the United States imposed new sanctions on Iran in the wake of the latest UN report offering evidence that Iran was working on a nuclear weapon. These sanctions included restrictions on the activities of Iran's central bank.

Dr. Massoumeh Torfeh, a London-based expert on Iran, told CBC News the attacks should be no surprise, as Iranian President Mahmoud Ahmadinejad has been fanning the flames of anti-British sentiment in his speeches for days. The demonstration outside the British compounds were advertised in the local media.

Erdbrink told CBC News that the students are supporters of Iran's hardline leaders, and have a tradition of demonstrating at the British Embassy, even hurling rocks and petrol bombs on some occasions.

"In the years that I have been reporting from here, I have never seen them actually enter the building, take down the British flag and replace it with an Iranian one," he said.

With files from The Associated Press