Foreign Secretary William Hague and the United Nations have condemned the "brutal" attack on a UN compound which killed around 20 people in Afghanistan.
Some seven UN staff are thought to be among the dead following the violence at the compound in Mazar-e-Sharif, in the north of the country, and police have arrested the suspected mastermind behind the attack.
The killings took place following a protest over the burning of the Koran in a US church.
Foreign Secretary William Hague said: "This was a brutal act which I utterly condemn. The work of the United Nations Assistance Mission in Afghanistan is of utmost importance. It is vital that they can carry out their work in a safe and secure environment. I call upon the Afghan government to investigate this incident and bring the perpetrators to justice."
Rawof Taj, deputy police chief in Balkh province, said the mastermind was one of more than 20 people arrested after the attack.
The Norwegian Defence Ministry named one of the victims as Lt Col Siri Skare, a 53-year-old female pilot. A Swede and four UN guards from Nepal were also killed, and a Romanian citizen is also said to have died. It is not believed that any of the dead are British.
A number of UN staff are believed to have been injured in the attack, including a Russian who was the head of the mission in Mazar-e-Sharif.
UN secretary general Ban Ki-moon, in Nairobi, Kenya, said: "This was an outrageous and cowardly attack against UN staff, which cannot be justified under any circumstances and I condemn in the strongest possible terms".
US president Barack Obama also condemned the incident and underscored the importance of the UN's work in Afghanistan. "We stress the importance of calm and urge all parties to reject violence and resolve differences through dialogue," he said.
The protesters were condemning a reported burning of the Koran at the Rev Terry Jones' church, Dove Outreach Centre, in Gainesville, Florida. Pastor Jones told the BBC that he was "absolutely not responsible" for Friday's atrocities. Instead he launched another attack on Islam saying: "We must take a serious, serious look at Islam. It's a violent religion that promotes acts of violence, I believe we need to bring this before the UN."