Shooting and bomb attack left at least 17 reported dead in Norway on Friday as a gunman disguised as a policeman opened fire at a youth camp and a bomb blast tore through government buildings.
Many were also reported wounded from the expolsion in central Oslo and the shooting at a summer school meeting of Prime Minister Jens Stoltenberg's ruling Labour Party on an island outside the capital.
Authorities were reeling, with police saying they had no clue who or what was behind the attack, but the gunman behind the shooting had been arrested.
The United States and European leaders immediately denounced the attacks and vowed solidarity with NATO member Norway -- an enthusiastic participant in international military missions that has forces in Afghanistan and is participating in Western air strikes in Libya.
Local media quoted officials as saying 10 people had been killed in the attack on a youth camp on Utoeya, an island just outside Oslo where Stoltenberg had been due to give a speech on Saturday to the 560 people attending.
"We have received information on the death of 10 people while seven have been wounded" in the shooting, police spokesman Bjoern Erik Sem-Jakobsen was quoted as saying by the Nettavisen news website.
"This figure is still not definitive but that is what we have been able to establish so far." Witnesses described scenes of panic and horror after the gunman, who media reports said was disguised as a police officer, opened fire on the youth gathering.
"I saw a lot of people running and screaming, I ran to the nearest building and hid under a bed," Emilie Bersaas, 19, told Britain's Sky News.
"It was very terrifying. At one point the shooting was very very close to the building, it actually hit the building, the people in the next room screamed."
"It is kind of unreal, especially in Norway," Bersaas said. "This is something we hear about happening in the US."
Police said they had detained the gunman and that his identity was known, but refused to say if he was a Norwegian citizen.
Local media quoted witnesses describing the gunman as northern European and armed with an automatic rifle, but the reports were not confirmed.
Norwegian police said they feared there could also be explosives on the island. Reports of the island shooting emerged shortly after a blast tore through the government quarter in central Oslo, home to the prime minister's office, other ministries and some of the country's leading media.
Police said a "bomb" had been behind the "powerful explosion". Stoltenberg was safe and there were no reports of other senior government officials being killed or wounded. The government was to hold a crisis meeting later Friday.
"We can confirm that we have seven dead and two have been seriously injured" in the bomb attack, a police spokesman told reporters at a briefing in Oslo. Several dozen were also wounded, police said.
"We have no main theory, we don't even have a working theory," a police official said separately. "We already have enough to do to get an understanding of the situation."
Police did say however that they believed the two attacks were connected. "There are good reasons to believe that there is a link between the events," police commissioner Sveinung Sponheim told reporters in Oslo.
Police also said that the gunman was probably linked to the bombing. Oslo's mayor Fabian Stang said the capital was struggling to come to terms with the idea that it had joined the list of cities targetted by bombers.
"Today we think about those people living in New York and London who have experienced this kind of thing," he told Sky.
"I do not think it is possible for us to understand what has happened today but hopefully we will be able to go on and that tomorrow Oslo will be a peaceful city again."
Images on Norwegian television showed the prime minister's office and other buildings heavily damaged, sidewalks covered in broken glass and smoke rising from the area.
A police spokesman said a vehicle had been seen driving at high speed in the area just before the explosion but did not confirm that the blast had been caused by a car bomb.
Police had sealed off the area and urged residents to stay in their homes. Stoltenberg gave a telephone interview to show that he was unharmed after the deadly blast, which he called a "serious situation".
"Even if one is well prepared, it is always rather dramatic when something like this happens," he said in the interview with a Norwegian TV station.
The EU condemned the attacks as "acts of cowardice" and the NATO chief denounced them as "heinous". US President Barack Obama called the attacks "a reminder that the entire international community has a stake in preventing this kind of terror from occurring."
Norway's intelligence police agency (PST) said in February that Islamic extremism was a major threat to the country, describing it as "our main priority and our main concern".
Norway, which counts some 500 troops in Afghanistan, has never suffered an attack at home by Islamic extremists. However, police last year arrested three Muslim men based in Norway suspected of planning an attack.
Norwegian F-16 fighter jets are also participating in air strikes in Libya, though the country has said it will withdraw its forces from the Libya operations on August 1.
The Norwegian capital is also a well-known symbol of international peace efforts, home to the Nobel Peace Prize and the birthplace of the 1993 Israeli-Palestinian Oslo Accords.