Prime Minister Jens Stoltenberg called the tragedy peacetime Norway's deadliest day.
"This is beyond comprehension. It's a nightmare. It's a nightmare for those who have been killed, for their mothers and fathers, family and friends," Stoltenberg told reporters Saturday.
Gun violence is rare in Norway, where the average policeman patrolling in the streets doesn't carry a firearm. Reports that the assailant was motivated by political ideology were shocking to many Norwegians, who pride themselves on the openness of their society. Indeed, Norway is almost synonymous with the kind of free expression being exercised by the youth at the political retreat.
King Harald V, Norway's figurehead monarch, vowed Saturday that those values would remain unchanged.
"I remain convinced that the belief in freedom is stronger than fear. I remain convinced in the belief of an open Norwegian democracy and society. I remain convinced in the belief in our ability to live freely and safely in our own country," said the king.
The monarch, his wife and the prime minister led the nation in mourning, visiting grieving relatives of the scores of youth gunned down. Buildings around the capital lowered their flags to half-staff. People streamed to Oslo Cathedral to light candles and lay flowers; outside, mourners began building a makeshift altar from dug-up cobblestones. The Army patrolled the streets of the capital, a highly unusual sight for this normally placid country.
The city center was a sea of roadblocks Saturday, with groups of people peering over the barricades wherever they sprang up, as the shell-shocked Nordic nation was gripped by reports that the gunman may not have acted alone. Police have not confirmed a second assailant but said they are investigating witness reports.
The queen and the prime minister hugged when they arrived at the hotel where families are waiting to identify the bodies. Both king and queen shook hands with mourners, while the prime minister, his voice trembling, told reporters of the harrowing stories survivors had recounted to him.
On the island of Utoya, panicked teens attending a Labour Party youth wing summer camp plunged into the water or played dead to avoid the assailant in the assault. A picture sent out on Twitter showed a blurry figure in dark clothing pointing a gun into the water, with bodies all around him.
The carnage hours earlier in Oslo, when a bomb rocked the city where the Nobel Peace Prize is awarded, left a square covered in twisted metal, shattered glass and documents expelled from surrounding buildings.
The dust-clogged scene after the blast reminded one visitor from New York of Sept. 11.
A 15-year-old camper named Elise who was on Utoya said she heard gunshots, but then saw a police officer and thought she was safe. Then he started shooting people right before her eyes.
Elise, whose father didn't want her to disclose her last name, 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.
At a hotel in the village of Sundvollen, where survivors of the shooting were taken, 21-year-old Dana Berzingi wore pants stained with blood. He said the fake police officer ordered people to come closer, then pulled weapons and ammunition from a bag and started shooting.
Several victims "had pretended they were dead to survive," Berzingi said. But after shooting the victims with one gun, the gunman shot them again in the head with a shotgun, he said.
Earlier, the police official who spoke on condition of anonymity said the attack "is probably more Norway's Oklahoma City than it is Norway's World Trade Center." Domestic terrorists carried out the 1995 attack, while foreign terrorists were responsible for the Sept. 11, 2001 attacks.
The United States, European Union, NATO and the U.K., all quickly condemned the bombing, which Britain's Foreign Secretary William Hague called "horrific" and NATO Secretary-General Anders Fogh Rasmussen deemed a "heinous act."
"It's a reminder that the entire international community has a stake in preventing this kind of terror from occurring," President Barack Obama said.
Obama extended his condolences to Norway's people and offered U.S. assistance with the investigation. He said he remembered how warmly Norwegians treated him in Oslo when he accepted the Nobel Peace Prize in 2009.
Britain's Queen Elizabeth II wrote to Norway's King Harald to offer her condolences and express her shock and sadness at the shooting attacks in his country.
A U.S. counterterrorism official said the United States knew of no links to terrorist groups and early indications were the attack was domestic. The official spoke on condition of anonymity because the investigation was being handled by Norway.
-Nordstrom reported from Stockholm. Associated Press reporters Bjoern H. Amland in Spundvollen, Norway, Nils Myklebost Oslo, Karl Ritter in Stockholm, Rita Foley in Washington, Paisley Dodds in London, and Paul Schemm in Tripoli, Libya, contributed to this report.