Twin shooting and bomb attacks left at least 87 dead as a Norwegian gunman disguised as a policeman opened fire at a youth camp and a bomb blast tore through government buildings in downtown Oslo.
"We have confirmation that at least 80 people are dead. We do not exclude a higher toll," police spokesman Are Frykholm told the media, speaking of the shooting spree a summer school meeting of Prime Minister Jens Stoltenberg's ruling Labour Party on Utoeya, an island outside the capital.
Police had earlier confirmed that seven people were killed as a powerful bomb ripped through central Oslo - where the prime minister's office and several government buildings are located - and nine were critically injured.
A 32-year-old Norwegian was arrested after the shooting spree. According to the TV2 channel, he has links to right-wing extremists and possessed two weapons registered in his name.
Stoltenberg said the culprits would not intimidate one of Europe's most peaceful countries.
"People have lived through a nightmare that very few of us can imagine," he said. "The coming days will show who is responsible and what kind of punishment they will get.
"The message to whoever attacked us, the message from all of Norway is that you will not destroy us, you will not destroy our democracy and our ideals for a better world."
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.
Stoltenberg had been due to give a speech on Saturday to the 560 people attending the youth camp on the island.
Witnesses described scenes of panic and horror after the gunman, who police said was disguised as a police officer but never worked for the police force, 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 is kind of unreal, especially in Norway... This is something we hear about happening in the US."
Another young survivor, Jorgen Benone, said: "People were hiding behind stones. I saw people being shot... I felt it was best to stay quiet, not to run into the open.
"I saw (the gunman) once just 20 to 30 metres away from me," Benone said, adding that he then swam to safety and was rescued by a boat.
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. Police said a "bomb" had been behind the "powerful explosion."
"There are good reasons to believe that there is a link between the events," police commissioner Sveinung Sponheim told reporters in Oslo.
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."
The prime minister's office and other buildings were heavily damaged, while sidewalks were covered in broken glass as smoke rose above the wreckage.
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.
UN chief Ban Ki-moon and the EU condemned the attacks 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.
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.