(CNN) -- A giant explosion rocked government buildings in Oslo, Norway, on Friday, state TV broadcaster NRK said, with at least one person confirmed dead.
Windows in several buildings had been blown out, and people were in the street bleeding, NRK said on its website. The cause of the blast remains unknown.
There are conflicting reports about whether a second blast followed the first, which occurred mid-afternoon in the center of the Norwegian capital.
A spokesman for Oslo police told CNN: "There has been a bomb explosion in the government area. At least one person is dead, and a number of people are injured. We don't have the exact number yet.
"We don't know if this comes from a terrorist action; we don't know yet. We don't know exactly how many explosions (there) were yet. Oslo center has been evacuated."
He confirmed that Prime Minister Jens Stoltenberg was not in his office at the time of the blast.
Linda Reinholdsen, a reporter for Norwegian state broadcaster NRK, said one explosion happened near the prime minister's office. Another hit near the Norwegian parliament, she told CNN.
Several buildings in Oslo were on fire, she said, and smoke was pouring from them.
Walter Gibbs, a journalist with Reuters, said he saw eight injured people, including two or three with serious wounds and one who looked dead.
Gibbs said he believes one explosion happened on an upper floor of a main government building. He said it blew out every window on the side of the building.
The blast also severely damaged the Oil Ministry and left it in flames, he said.
Reuters reported that Stoltenberg was safe.
Stoltenberg, who has been prime minister since October 2005, heads a coalition government made up of the Labour Party, the Socialist Left Party and the Centre Party.
A U.S. official said it was too soon to tell what caused the explosion or whether it was a terrorist attack. The possibility of terrorism is always a concern because of the ongoing threat from al Qaeda to launch attacks in Europe, the official said.
A representative for Oslo University Hospital confirmed that seven patients were being treated there after the explosion.
"I am not aware whether they are major or minor injuries. We have spoken to the other hospitals in Oslo, and in total, we are sending 22 ambulances and five helicopters. We currently have no confirmation of any deaths."
Nick Soubiea, an American-Swedish tourist in Oslo, told CNN he was less than 100 yards from the blast, which he described as deafening.
"It was almost in slow motion, like a big wave that almost knocked us off our chairs," he said. "It was extremely frightening."
He said the streets were crowded with people trying to get away from the center of the city. "There are people running down the streets, people crying, everyone's on their cell phones calling home," he said.
Jon Martin Larsen, head of media for the Norwegian Red Cross, told CNN: "The Norwegian Red Cross has established its own crisis team and is in contact with the municipality of Oslo and the police, ready to assist with whatever they need of first aiders, rescue teams, ambulance or caretakers either in the city center or at the hospitals."
A hotel worker at Oslo's Grand Hotel, about a five-minute walk from the government building, said everyone in the hotel felt and heard the explosion, which felt like someone was shaking the entire building.
"It's crazy," she said, not wanting to be identified because she is not authorized to speak to the media on behalf of the hotel. "This happens in the big world, not in Oslo. I'm shocked."
Vivian Paulsen, media adviser for the Norwegian Red Cross, lives 20 minutes away from the center of Oslo in the northern outskirts of the city. She said she heard a "huge blast."
"I heard the big bang, I didn't think it was anything serious. I can still see smoke coming up from the place," she said, watching from her apartment balcony. She also heard sirens and ambulances.
As for Oslo, she said what others have been saying: Events like this don't happen in the northern European capital.
"There's occasional arrests of terror suspects we read about in the paper, or people planning something."
CNN's Joe Sterling, Moni Basu, Chelsea Bailey and Claudia Rebaza contributed to this report.