The United Nations mission in Afghanistan was thrown into crisis last night after two staff were beheaded and at least six others killed when a mob overran its regional headquarters.
Police in Mazr-e Sharif - a normally peaceful city in the north - said a man and a woman were decapitated after demonstrators overwhelmed police guarding their compound. The other six, thought to be Gurkha guards, were shot.
The attack - one of the worst in the UN's history in Afghanistan - comes just days after president Hamid Karzai declared the city was safe enough for Afghan forces to assume control from Nato.
UN Secretary General Ban Ki-moon described it as "an outrageous and cowardly attack".
US President Barack Obama also condemned the attack "in the strongest possible terms", saying the work of the UN "is essential to building a stronger Afghanistan".
The top UN representative in Afghanistan, Staffan de Mistura, was flying to the scene last night. Officials in Kabul said they were struggling to get information as the UN office had been "completely wiped out".
Eyewitnesses said hundreds of protesters met at the city's main mosque for Friday prayers, and then marched on the UN compound to demonstrate against an extremist American pastor, Terry Jones, who burned a Koran last month.
The vast Blue Mosque - one of the holiest sites in Islam - is less than a mile from the UN office.
"The mullahs told us to come to the Blue Mosque for Friday prayers to protest against the burning of the Koran," said Abuzar, 28, a madrassa student.
"We were more than 2,000 people, but we didn't want to attack the UN compound. I don't know how the situation changed.
"A few people took the guns from the guards and went inside. Then the guards on the roof started shooting and everyone escaped."
Police said the protest turned violent at around 4pm, when men armed with knives stormed the Afghan guards and then used their assault rifles to break in. At least three protesters were shot dead and 20 injured. Twenty seven were later arrested.
"Around ten men entered the UN compound and started firing," said Lal Mohammad Ahmadzai, the regional police spokesman. "Eight foreigners have been killed and one other injured."
UN premises in Afghanistan are usually guarded by Afghan police on the outside, and Nepalese Gurkhas inside. The Gurkhas were introduced after a suicide squad overwhelmed police guards at a guesthouse in Kabul, in 2009, killing six and injuring nine people.
That attack prompted more than 600 international staff to leave Afghanistan for offices in the Emirates. Gurkhas successfully repelled a similar insurgent attack last year in Herat.
Officials refused to comment on how the various UN agencies would respond to the latest violence.
UN Staff across the country were on lockdown last night amid fears of more attacks and sources told The Scotsman that mass evacuation plans were being "dusted off".
But Kieran Dwyer, director of communications for the UN mission in Afghanistan, said: "The UN is here to stay. We're here to work with the people to help them achieve peace, and this sort of thing just highlights how important that is."
Pastor Terry Jones, a Christian fundamentalist based in Florida, had originally planned to burn a Koran on the anniversary of the 11 September attacks last year, but he was persuaded not to by US president Barack Obama.
However, he went ahead and burned Islam's sacred book, after what he called "putting it on trial", last month.
Last night Pastor Jones described yesterday's massacre as "tragic and criminal".
"The time has come to hold Islam accountable," he said.
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