A massive bomb in the centre of the Norwegian capital, Oslo, has killed at least seven people and damaged government buildings.
The bomb was followed on Friday by a fatal shooting incident two hours later at a Labour Party youth camp at Utoeya, an island south of Oslo.
At least 10 people are reported to have been killed when a man dressed as a police officer fired indiscriminately.
The toll is expected to rise. Police say the bomb and the shooting are linked.
State media said one man was later arrested.
Justice Minister Knut Storberget has identified the suspect as Norwegian.
Deputy Oslo police chief Sveining Sponheim said the detained man was aged 32 and ''ethnic Norwegian''.
Police say there are undetonated explosives on the island.
Witnesses have told the BBC they have seen bodies in the water around the island.
One witness said he had seen more than 20 bodies on the island, but police have not confirmed this.
Prime Minister Jens Stoltenberg, who was due to visit the camp on Saturday, told TV2 television that the situation in Utoeya was critical.
About 560 people were attending the camp.
He also said that all cabinet ministers in the coalition government seemed to be safe.
''This is very serious,'' he said.
Mr Stoltenberg said that police had told him not to say where he was speaking from.
The government is to hold a crisis meeting.
Central city sealed off
National broadcaster NRK says all roads into central Oslo have been closed.
Reuters news agency is reporting that soldiers are taking up positions around central Oslo.
Police have confirmed that the explosion was caused by a bomb. The BBC reports at least 15 people were injured.
Assistant Chief Constable of Oslo Egil Vrekke told the BBC:
''We have cordoned off large areas. There are bomb experts at the scene investigating whether there are other devices in the area.''
Oslo University hospital said 10 people had been taken there for treatment, some with serious injuries.
The bombing occurred at around 3.30pm on Friday.
Windows of the prime minister's office were shattered by the bomb blast.
Mr Stoltenberg was reported to have not been in his office at the time of the blast.
The office of a tabloid newspaper, VG, was also affected.
The offices of the Norwegian Red Cross were close to the site of the explosion.
Red Cross employee Oistem Mariyam said the area was very busy when the bomb went off.
However, Friday was a public holiday in Norway so the offices were not as busy as they might usually have been.
Nation in shock
Foreign Minister Jonas Gahr Store told the BBC the country was in ''deep, deep shock''.
''Norway is today dealing with a double attack on its democracy, on its government buildings and on its finest youth wanting to engage in politics,'' he said.
In the main, Norway has stayed away from international conflict.
Though a long-standing member of NATO, Norway has only recently increased its involvement in military missions in countries such as Afghanistan or Libya.
Consequently, the BBC reports its foreign policy has not made the country many enemies.
The Royal Family is free to move about with limited security, both in the cities, as well as on holiday.
Politicians and business leaders mix freely with ordinary people in a manner rarely seen elsewhere in the world.
Few people have secret addresses and telephone numbers. Politicians, business executives and other celebrities often include their private telephone numbers and home addresses on their business cards.
Even their salaries and the size of their fortunes are made public once a year.
Neighbouring Sweden was targeted in a suicide bombing last December when a man whose family fled from Iraq to Sweden in 1991, blew up himself and his car in a deserted street in central Stockholm, injuring two people.
Swedish Prime Minister Olof Palme was murdered in 1986.