Police in Norway say at least 84 were killed in shootings at Utoeya Island where a Labour Party youth camp was attacked on Friday.
Police say the killings are of "catastrophic dimensions". Many other people were wounded.
A gunman disguised as a policeman opened fire at the camp two hours after a bomb explosion in central Oslo in which at least seven people died.
The blast ripped through government buildings, including the prime minister's office. The prime minister was not there at the time.
Friday was a public holiday in Norway so the offices were not as busy as they might usually have been.
A man has been charged in connection with both the shooting and the bomb attack.
He has been named as Anders Behring Breivik, 32, an ethnic Norwegian.
Oslo police say the suspect's website indicates he is of a right-wing Christian fundamentalist orientation.
The Norwegian media say the man describes himself on his Facebook page as conservative, Christian, single, and interested in hunting and games such as World of Warcraft and Modern Warfare 2.
The BBC reports that police are investigating whether the attacks were the work of one man or whether he had help.
Eyewitnesses described how a tall, blonde man dressed as a policeman opened fire indiscriminately on Utoeya Island, prompting camp attendees to jump into the water to try and escape the hail of bullets.
Some of the teenagers were shot at as they tried to swim to safety.
A Norwegian journalist, Ole Torp told the BBC that the gunman had taken the ferry boat from the mainland to the island posing as a police officer and saying he was there to do research in connection with the bomb blasts.
''He asked people to gather round and then he started shooting,' he said.
Hundreds of young people were attending the summer camp, which was organised by the ruling Labour Party and police said more dead could be found as rescue teams scoured the surrounding waters.
Address by PM
Prime Minister Jens Stoltenberg delivered a televised address to the nation.
''We've all been shaken by the evil that struck us so brutal. I have a message to whoever attacked us: you will not destroy us. You will not destroy our democracy,'' he said.
''We are a small nation and a proud nation. No-one will bomb us to silence no-one will shoot us to silence,'' he said.
The Prime Minister later gave a news conference at which he said the shooting had turned a "youth paradise into a hell".
"Many of those who lost their lives were persons I know. I know the young people and I know their parents," he told reporters. "And what hurts more is that this place where I have been every summer since 1979, and where I have experienced joy, commitment and security, has been hit by brutal violence -- a youth paradise has been transformed into a hell."
"Not since World War II has our country seen a greater crime," he said.
Mr Stoltenberg also revealed that several members of his administration were among the dead in the bomb attack in Central Oslo.