A powerful bomb ripped open several Oslo buildings, including the prime minister's office, killing at least two people and possibly many more.
Several people also were injured, as the blast shattered windows and coated the street with documents.
And Sky News has reported that shots were fired at a children's summer camp close by, which was being attended by 700 people.
Sky also reported that four were shot dead at a youth event on the outskirts of the city
A Norwegian government official says there are people trapped in the buildings hit by a bomb blast at the government headquarters in Oslo.
State Secretary Kristian Amundsen told BBC that "there are still people in the buildings" at the blast site. He declined to elaborate, saying: "I can't go into details.
He said this situation is the worst his country has seen.
Prime Minister Jens Stoltenberg is safe, government spokeswoman Camilla Ryste told The Associated Press. A senior adviser to the prime minister, Oivind Ostang, said Stoltenberg was working at home Friday and was not in the building.
Norwegian news agency NTB says police confirm the Oslo explosion was caused by a bomb. The country's public broadcaster, NRK, said two people were confirmed killed.
"There has been one or several powerful explosions in the government district in Oslo," Oslo police said in a brief statement.
"So far, police cannot say anything about the scope of the damage, aside from that there's been one or several explosions."
An Oslo police official, who would not give her name because she was not authorized to speak to the media, said "there are several people injured."
She said the cause of the explosion was not clear. Asked whether police suspected a terror attack, she said: "It's too early to speculate in that."
NRK showed video of a blackened car lying on its side amid the debris.
Most of the windows in the 20-floor high-rise were blown out, and the bottom floor appeared to be gutted. Nearby offices were also heavily damaged and evacuated, including those housing some of Norway's leading newspapers and news agency NTB.
Witness Ole Tommy Pedersen was standing at a bus stop about 100 meters (yards) from the high-rise at around 3:30 p.m. (1330 GMT) when the explosion occurred.
"I saw three or four injured people being carried out of the building a few minutes later," Pedersen told AP.
An AP reporter who was in the NTB office said the building shook from the blast and all employees evacuated as the alarm went off. Down in the street, he saw one person with a bleeding leg being led away from the area.
The government building houses the offices of the prime minister and his administration. Several ministries are in surrounding buildings.
The blast comes as Norway grapples with a homegrown terror plot linked to al-Qaida. Two suspects are in jail awaiting charges.
Last week, a Norwegian prosecutor filed terror charges against an Iraqi-born cleric for threatening Norwegian politicians with death if he is deported from the Scandinavian country. The indictment centered on statements that Mullah Krekar - the founder of the Kurdish Islamist group Ansar al-Islam - made to various news media, including American network NBC.
Terrorism has also been a concern in neighboring Denmark since an uproar over cartoons of the Prophet Muhammad six years ago. Danish authorities say they have foiled several terror plots linked to the 2005 newspaper cartoons that triggered protests in Muslim countries. Last month, a Danish appeals court on Wednesday sentenced a Somali man to 10 years in prison for breaking into the home of the cartoonist.
Independent News Service