A "catastrophic" shooting on an island outside Norway's capital Friday killed at least 80 people, police said early Saturday, much higher than the initial count released.

Police told reporters they had discovered many more victims after initially reporting the death toll at 10 at the youth camp of Norway's Labour Party at Utoya Island, outside the capital city.

The usually peaceful nation was reeling after two deadly attacks Friday: the shooting and a deadly bombing that damaged a government building in downtown Oslo. The explosion left seven people dead and nine or 10 seriously injured, police said.

According to a Reuters report, police director Oystein Maeland said the island attack had reached "catastrophic dimensions."

"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," Maeland said. "It goes without saying that this gives dimensions to this incident that are exceptional."

Maeland also cautioned that the death toll from the island shooting could increase, noting that many were badly wounded in the attack.

'I saw many dead people'

A 15-year-old camper named Elise said she heard gunshots, but then saw a police officer and thought she was safe. Then he started shooting people right before her eyes.

"I saw many dead people," said Elise, whose father, Vidar Myhre, didn't want her to disclose her last name. "He first shot people on the island. Afterward he started shooting people in the water."

Elise said she hid behind the same rock that the killer was standing on. "I could hear his breathing from the top of the rock," she said. She said it was impossible to say how many minutes passed while she was waiting for him to stop.

A shooter killed 80 people at a camp on Utoya island in Norway. Officials found youth hiding along with the bodies of shooting victims. Some are still missing, officials said. (ABC Nyheter)

A suspect was arrested in connection with both attacks. Though police did not release his name, Norwegian national broadcaster NRK identified him as Anders Behring Breivik, 32, and said police searched his Oslo apartment overnight. NRK and other Norwegian media posted pictures of the blond, blue-eyed Norwegian.

A police official said the suspect appears to have acted alone in both attacks, and that "it seems like that this is not linked to any international terrorist organizations at all." The official spoke to The Associated Press on condition of anonymity because that information had not been officially released by Norway's police.

Norwegian police found undetonated explosives on the island of Utoya, which an official said a police bomb disposal team and military experts were working on disarming. That official also spoke to The Associated Press on condition of anonymity.

Prime Minister Jens Stoltenberg, whose office is in the building that was bombed, said Norway will show the world that its democracy will only get stronger.

"You will not destroy us. You will not destroy our democracy and our ideals for a better world," 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. No one will ever scare us from being Norway. This evening and this night, we will take care of each other, comfort each other, talk to each other, and stand together.

"We must never stop standing up for our values."

Stoltenberg said Norway's answer to violence is more democracy, "but not more naiveté."

Acting police chief Sveinung Sponheim said the suspect, who he said was dressed in a police uniform when he opened fire into a crowd of youths, had been observed in Oslo before the explosion there.

Aerial images broadcast by Norway's TV2 showed members of a SWAT team dressed in black arriving at the island in boats and running up the dock. Behind them, people stripped down to their underwear swam away from the island toward shore, some using flotation devices.

In Oslo, police said the explosion was caused by "one or more" bombs, but declined to speculate on who was behind the attack. They later sealed off the nearby offices of broadcaster TV 2 after discovering a suspicious package.

Rail traffic to the airport was shut down, and travellers were advised extended border controls were in place, meaning stricter security and passport controls.

Stoltenberg appealed to people not to use their mobile phones to avoid overloading the network. Officials have asked people with O-type blood to donate.

'Shell-shocked'

Norwegian journalist Ketil Stensrud took to social media to keep people updated on the attacks.

"I was shell-shocked," he said. "I mean, you keep hearing about terrorist attacks all over the world … You don't expect this sort of thing to happen in your own backyard."

"We were actually under attack."

Stensrud said the epicentre of Norwegian politics is "just absolutely shattered and broken to bits. It looked like an absolute war zone."

Adrienne White, an Ottawa woman visiting Oslo, was three or four blocks from the explosion.

"It was totally, completely unexpected," she said. "It's so safe, it's just a huge shock."

White was in a tourist area, she said, when she heard the explosion. It sounded to her like a series of shotgun pops, she said.

"Everyone sort of looked up and a huge cloud — of debris and smoke and ash — a huge cloud [was] just going across the sky.

"Everyone sort of stopped dead in their tracks and were sort of stunned."

White said she didn't realize it was an explosion until she noticed all the glass from the windows in buildings around her had smashed.

"There was a huge piece of iron ... like four feet of an iron beam had smashed into a sign," she said. "It was pretty lucky no one got hit just by the debris."

Other buildings damaged house government offices and the headquarters of some of Norway's leading newspapers.

The explosion occurred at 3:30 p.m. (1330 GMT), as Tommy Pedersen was waiting for a bus after leaving work at a news agency.

"I heard a big explosion and a big white cloud of ... dust was coming out from the main entrance of the government building," he told CBC News Network.

"Then for me it sounded just like a wind, just the wind … but I quickly realized that the whole first floor, which is a concrete building … the whole first floor was torn out," he said.

The United States, European Union, Canada, NATO and the U.K., all quickly condemned the bombing.

Prime Minister Stephen Harper said he's shocked and horrified at the two violent attacks in Norway. Harper said that he was "intensely saddened to learn of the attacks in Oslo and Utoya."

"We deeply regret the loss of life and injuries resulting from the explosion which occurred today in the government quarters, where the prime minister’s office and other government offices are located," he said in a statement.

"Canada condemns these barbarous and senseless acts of violence," he said, expressing condolences to Stoltenberg and the people of Norway.

Defence Minister Peter MacKay said the attacks speak "to the volatility everywhere and anywhere when people who are so driven, so ideological, so willing to kill indiscriminately."

"This reminds us to be vigilant. It reminds us that we have a broader responsibility outside our own country to work in the international community and it does overlap with our efforts in Afghanistan. Clearly the training mission … will allow Canada to continue to make a very positive contribution to what we hope will be a more peaceful and more secure world," he said on CBC's Power & Politics.

With files from The Associated Press