Britain on Tuesday warned of “serious consequences” after Iranian protesters entered the British embassy compound in Tehran, replaced the missions flag with an Iranian flag and demanded that the British ambassador leave the country immediately.

CNN’s Shirzad Bozorgmehr noted Tuesday’s incident - which started with about 1,000 protesters gathering in front of the embassy - came on the one year anniversary of the killing of nuclear physicist Majid Shahriari, a top scientist and senior manager of Iran’s nuclear program, who died in a Tehran bombing for which Iran blames foreign intelligence services.

U.S. and allies such as the U.K. have accused Iran – which denies it - of seeking to develop atomic weapons under the guise of a civilian nuclear program.

The event was short-lived, lasting less than 2 hours before protestors were cleared out by police, the Associated Press reported

As reported in Bloomberg, U.K. Foreign Secretary William Hague said he spoke to Salehi “to protest in the strongest terms” and to “demand immediate steps to ensure the safety” of the embassy staff.

Hague said that Salehi said he was sorry for what happened.

U.K. Prime Minster David Cameron said in a statement that all embassy personnel have been accounted for after the “outrageous and indefensible,” attack.

“The Iranian Government must recognize that there will be serious consequences for failing to protect our staff,” Cameron said.