A bomb killed seven people in Norway's capital Oslo today and a gunman opened fire killing at least nine at a youth camp on an island, police said.
One witness said he saw 20 dead at the youth camp, but police said they had no confirmation of the number of deaths on the island.
Police said a man had been arrested and was being questioned by Oslo police, who believe he also has knowledge of the Oslo bomb
A witness said several army soldiers had taken up position around the centre of the city.
With police advising people to evacuate central Oslo, apparently in fear of more attacks, prime minister Jens Stoltenberg told Norwegian TV2 television in a phone call that the situation was "very serious".
He said that police had told him not to say where he was speaking from.
The bomb ripped through the main government building in the Norwegian capital at 3.30pm (Irish time), killing seven people and injuring many more.
"It exploded - it must have been a bomb. People ran in panic...I counted at least 10 injured people," said Kjersti Vedun, who was leaving the area of the blast in Oslo.
Shortly afterwards, a gunman opened fire at the island of Utoeya, approximately one hour north-west of Oslo, where Mr Stoltenberg's Labour party youth section's annual gathering was taking place.
Daily newspaper VG said on its website a man dressed as a policeman had been shooting wildly and had hit many people.
Norwegian commercial broadcaster TV2 said several people had been killed in the shooting spree.
There was no clear claim of responsibility and while the attacks appeared to bear the hallmarks of an Islamist militant assault, analysts said it was too early to draw any conclusions.
Nato member Norway has been the target of threats before over its involvement in conflicts in Afghanistan and Libya.
The attack came just over a year after three men were arrested on suspicion of having links to al-Qaeda and planning to attack targets in Norway. It came also less than three months after US forces killed Osama bin Laden in a raid on his hideout in Pakistan.
A Department of Foreign Affairs spokesman said it had received no reports of any Irish injuries. The Irish embassy in Oslo is situated close to the location of the bomb blast but was not damaged.
Violence or the threat of it has already come to the other Nordic states: a botched bomb attack took place in the Swedish capital Stockholm last December and the bomber was killed.
Denmark has received repeated threats after a newspaper published cartoons of the Prophet Mohammad in late 2005, angering Muslims worldwide.
The Oslo blast tore at the facade of the 17-storey central government building, blowing out most of the windows and scattering shards of metal and other debris for hundreds of metres.
The building of a publisher which recently put out a translation of a Danish book on the Mohammad cartoon controversy was also affected, but was apparently not the target.
The blast scattered debris across the streets and shook the entire city centre.
"This is a terror attack. It is the most violent event to strike Norway since World War Two," said Geir Bekkevold, an opposition parliamentarian for the Christian Peoples Party.
The district attacked is the very heart of power in Norway, with several other key administration buildings nearby.
Nearby ministries were also hit by the blast, including the oil ministry, which was on fire. Nevertheless, security is not tight given the lack of violence in the past.
The failed December attack in Stockholm was by a Muslim man who grew up in Sweden but said he had been angered by Sweden's involvement in the Nato-led force in Afghanistan and the Prophet Mohammad cartoons.
That attack was followed weeks later by the arrest in Denmark of five men for allegedly planning to attack the newspaper which first ran the Mohammad cartoons.
In July 2010, Norwegian police arrested three men for an alleged plot to organise at least one attack on Norwegian targets and said they were linked to individuals investigated in the United States and Britain.
John Drake, senior risk consultant at London-based consultancy AKE, said: "It may not be too dissimilar to the terrorist attack in Stockholm in December which saw a car bomb and secondary explosion shortly after in the downtown area.
"That attack was later claimed as a reprisal for Sweden's contribution to the efforts in Afghanistan." Political violence is virtually unknown in a country known for awarding the Nobel Peace Prize and mediating in conflicts, including in the Middle East and Sri Lanka.
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Reuters