A passer-by comforts an injured woman after a powerful explosion rocked central Oslo.

Oslo - Twin shooting and bomb attacks left at least 87 dead in western Europe's deadliest carnage since the 2004 Madrid bombings as a Norwegian gunman opened fire at a youth camp and a bomb tore through central Oslo.

The suspect was a 32-year-old Norwegian who posted anti-Muslim rhetoric online, police commissioner Sveinung Sponheim told the NRK television channel, but added: “It's too early to say if this was a motive behind the act.”

Norwegian media named him as Anders Behring Breivik, but police refused to confirm the information.

Police voiced fears that the toll could rise as they searched for victims of the shootings at a summer school meeting organised by Prime Minister Jens Stoltenberg's ruling Labour Party on Utoeya, an island outside the capital.

Security was meanwhile tightened across potential target sites in the capital, police said Saturday, but they lifted an advisory that had urged residents to stay home.

“We have confirmation that at least 80 people are dead. We do not exclude a higher toll,” police spokesman Are Frykholm told AFP, speaking of the shooting spree on the island.

Police had earlier confirmed that seven people were killed when a powerful bomb ripped through central Oslo - where the prime minister's office and several government buildings are located - and nine were critically injured.

According to the TV2 channel, the arrested suspect has links to right-wing extremists and possessed two weapons registered in his name.

Other Norwegian media reported that he described himself on his Facebook page as “conservative”, “Christian”, and interested in hunting and computer games like World of Warcraft and Modern Warfare 2.

Prime Minister Stoltenberg said Norway, one of Europe's most peaceful countries, would not be intimidated.

“People have lived through a nightmare that very few of us can imagine,” he said. “The coming days will show who is responsible and what kind of punishment they will get.

“The message to whoever attacked us, the message from all of Norway is that you will not destroy us, you will not destroy our democracy and our ideals for a better world.”

Western leaders denounced the attacks and vowed solidarity with NATO member Norway, which has forces in Afghanistan and is participating in air strikes in Libya.

Stoltenberg had been due to give a speech on Saturday to the 560 people attending the youth camp on the island.

Witnesses described scenes of panic and horror after the gunman, who police said was wearing a police uniform but had never worked for the police force, opened fire on the youth gathering.

“I saw a lot of people running and screaming, I ran to the nearest building and hid under a bed,” Emilie Bersaas, 19, told Britain's Sky News.

“It is kind of unreal, especially in Norway... This is something we hear about happening in the US.”

Another young survivor, Jorgen Benone, said: “People were hiding behind stones. I saw people being shot... I felt it was best to stay quiet, not to run into the open.

“I saw (the gunman) once just 20 to 30 metres away from me,” Benone said, adding that he then swam to safety and was rescued by a boat.

Norwegian police said they feared there could also be explosives on the island.

Reports of the island shooting emerged shortly after a blast tore through the government quarter in central Oslo. Police said a “bomb” had been behind the “powerful explosion”.

“There are good reasons to believe that there is a link between the events,” police commissioner Sponheim told reporters in Oslo.

Mayor Fabian Stang said the capital was struggling to come to terms with the idea that it had joined the list of cities targeted by bombers.

“Today we think about those people living in New York and London who have experienced this kind of thing,” he told Sky.

The prime minister's office and other buildings were heavily damaged, while pavements were covered in broken glass as smoke rose above the wreckage.

A police spokesman said a vehicle had been seen driving at high speed in the area just before the explosion, but did not confirm that the blast had been caused by a car bomb.

Police had sealed off the area and urged residents to stay in their homes.

UN chief Ban Ki-moon and the EU condemned the attacks and the NATO chief denounced them as “heinous”.

US President Barack Obama called the attacks “a reminder that the entire international community has a stake in preventing this kind of terror from occurring.”

The Norwegian capital is a well-known symbol of international peace efforts, home to the Nobel Peace Prize and the birthplace of the 1993 Israeli-Palestinian Oslo Accords.

The attacks were the worst in western Europe since the 2004 train bombings in Madrid, which left 191 dead and nearly 2 000

injured. - Sapa-AFP