Norwegians will defend themselves by showing they are not afraid of violence and by participating more broadly in politics, he told reporters.
"It's absolutely possible to have an open, democratic, inclusive society, and at the same time have security measures and not be naive," he said.
Stoltenberg underlined his commitment to openness, defending freedom of thought even if includes extremist views such as those held by the 32-year-old Norwegian who confessed to Friday's bomb blast at government headquarters and to the shooting massacre at a Labor Party youth camp hours later. At least 76 people were killed.
"We have to be very clear to distinguish between extreme views, opinions - that's completely legal, legitimate to have. What is not legitimate is to try to implement those extreme views by using violence," he said.
"I think what we have seen is that there is going to be one Norway before and one Norway after July 22," he said. "But I hope and also believe that the Norway we will see after will be more open, a more tolerant society than what we had before."
He later announced the independent commission, saying "it is important to be able to clear up all questions about the attack in order to learn from what happened."
The parties in parliament have agreed to earmark an unspecified amount of money to cover some of the funeral costs, he said. A national memorial will also be created.
The vicious attacks in the placid, liberal country have left Norwegians appalled and shaky, but determined to move forward. Some government workers were planning to return to work in their offices in the buildings where the bomb blasts blew out most windows.
Denmark said Wednesday a 43-year-old Danish woman, Hanne Balch Fjalestad, had died in the attacks, marking the first confirmed foreign death.
She was working as a first aid medic at Utoya island. She leaves behind four children, including a 20-year-old daughter, Anna, who survived the island shooting.
Anders Behring Breivik has confessed to the attacks, saying he was trying to save the Western world from Muslim colonization.
Earlier, the leader of Norway's Delta Force defended the special operations team, saying the breakdown of a boat didn't cause a significant delay in efforts to reach the island.
Police have come under close scrutiny over how long it took them to reach the island after first reports of shots being fired at the island youth camp Friday. Although the island is only about 25 miles (40 kilometers) from the Norwegian capital, police needed 90 minutes to get to the scene.
A media helicopter was already hovering over the island when police arrived. Marius Arnesen, a cameraman for broadcaster NRK who shot video of the massacre at Utoya island, told The Associated Press that his helicopter arrived some time between 6 p.m. and 6:10 p.m. Police say got to the island at 6:25 p.m.
Police were already grappling with the wide damage inflicted in the downtown government quarter. When word of the shooting came, police drove rather than take a helicopter because the crew of the sole chopper available to them was on vacation. Then the first boat they tried to take to the lake island broke down.
Anders Snortheimsmoen told reporters the team immediately jumped into another, better boat. He said his team arrived at the harbor at the same time as local police and the boat mishap caused no delay.
Once they arrived, Snortheimsmoen told the AP, his officers nearly shot Breivik because they feared he might be wearing an explosive belt. The decision was made by a "very narrow margin," he said.
Police later offered a detailed account of Breivik's arrest, saying he surrendered readily. Squad leader Haavard Gaasbakk told reporters police yelled at the gunman when they drew near and the shooter lay down his weapon and held his hands high over his head.
"When we get closer to the place where there's shooting we started to use our voices, yelling 'armed police' to draw the attention to us," Gaasbakk said.
"We come to a forested aread and the suspect stands there right in front of us with his hands high above his head," Gaasback said.
Justice Minister Knut Storberget praised the team at the news conference, saying it had helped "limit the tragedy."
Norwegian media are suggesting that police knew Breivik's identity even before they reached the island, tracing him through a rental car company from which he rented the panel van in which the bomb was planted.
Dag Andre Johansen, Scandinavian CEO of Avis car rental company, told the AP that Breivik had rented two vehicles, including a Volkswagen Crafter van. He said police contacted the company after the bombing and got Breivik's identity confirmed. But he declined to say whether that contact came before Breivik was arrested on the island.
Many in Oslo felt a new twinge of worry on Wednesday morning when parts of the capital's rail and bus complex was evacuated because of a suspicious abandoned suitcase. Police later said no explosives were found and that the evacuation order had been lifted. The Norwegian news agency NTB said a bus driver turned in the alarm after seeing a passenger leave the suitcase and walk into the station at a fast clip.
Police officially released the first four names of victims on Tuesday, and Norwegian media published the names and photos of some of the other victims. At least some were immigrants or their descendants.
Tens of thousands of Norwegians have rejected the suspect's anti-immigrant rhetoric, laying thousands of flowers around the capital in mourning. Entire streets were awash in flowers, and Oslo's florists ran out of roses.
Norway's Crown Prince Haakon and Foreign Minister Jonas Gahr Stoere attended a packed memorial Tuesday in the World Islamic Mission mosque in Oslo. After the ceremony, Pakistani-born Imam Najeeb ur Rehman Naz said the massacre had brought Norwegian residents of all backgrounds closer together.
"Everyone realizes that terrorism and this kind of activity doesn't have anything to do with any religion," he told the AP. "They are individuals who can be found in any community who don't represent the majority at all."
Many of those killed were involved in the governing Labor Party, which suspect Breivik rails against in his manifesto for allowing Muslims to immigrate to Norway.
Richard Steed in Copenhagen contributed to this report.