Norway's Prime Minister said that his "childhood paradise had been transformed into hell" after a gunman killed at least 84 people at a youth camp on the island of Utoya and seven more died in a bombing attack on Oslo.
A man has been arrested and provisionally charged by police on two counts of terrorism.
At a press conference on Saturday morning PM Jens Stoltenbeg said that the attacks were a "national tragedy".
"Never since the Second World War has our country been hit by a crime on this scale," he said.
At least 85 people are now known to have been killed on the island, which is around 30km from Oslo, after a blond, blue-eyed Norwegian man dressed as a police officer opened fire on young people attending a summer youth camp for the country's Labour party.
Panicked victims reportedly tried to hide and run into the water surrounding the island, but were fired upon from the shore in an attack that may have lasted for more than an hour before a man was apprehended.
The shootings followed an explosion in central Oslo that killed at least seven people. Police said that the bomb was set off by the same suspect.
Initially many assumed that the attacks were the work of Al Qaeda or an affiliated organisation, as with similar incidents in Europe in recent years. At least one newspaper in the UK even ran a headline to that effect.
Norwegian broadcasters have named the suspect as 32-year-old Anders Breivik, a Norweigen man. It was reported by police that the man, who is a "Christian fundamentalist", had right-wing and anti-Muslim views and was "co-operating" with the investigation, wanted the chance to explain his reasons for the massacre.
Breivik has been charged under terrorism laws on two separate counts, representing the two incidents, police told the BBC. He will now be questioned for up to three days before appearing in court.
Police said initially that 10 people had been killed on the island but the death toll rose overnight as more victims were discovered.
Police director Oystein Maeland said:
"It's taken time to search the area. What we know now is that we can say that there are at least 80 killed at Utoya," Mr Maeland said. "It goes without saying that this gives dimensions to this incident that are exceptional."
Police have warned that the death toll is likely to rise further, and said that many others were still critically injured.
Prime Minister David Cameron said that the UK would do all it could to help.
"These attacks are a stark reminder of the threat we all face from terrorism. I have called Prime Minister Stoltenberg this evening to express my sincere condolences and to let him know that our thoughts are with the Norwegian people at this tragic time."
"I have offered Britain's help, including through our close intelligence cooperation. We will work with Norway to hunt the murderers who did this and prevent any more innocent deaths. We can overcome this evil, and we will."
The two attacks represent the worst combined day of violence in Western Europe since the 2004 bombings in Madrid that killed 191 people.