A huge blast shattered the main government office in central Oslo on Friday, reportedly killing two and injuring several, but Norway's Prime Minister Jens Stoltenberg was reported to be safe.
There was no confirmation the explosion was caused by a bomb nor any claim of responsibility, though Nato member Norway has been the target of threats, if not bombs, before, notably over its involvement in conflicts in Afghanistan and Libya.
Here are some recent militant attacks and plots in Europe.
* Madrid bombings:
-- On March 11, 2004 10 bombs hidden in sports bags exploded on four packed commuter trains at the height of the morning rush hour in Madrid, killing 191 people and wounding 1,700.
-- The bombings were Europe's deadliest Islamist militant attack. Fernando Reinares, a leading expert on militant Islamist violence in Spain, says they probably were instigated by militants hiding in north Waziristan, a region of northwest Pakistan believed to harbour al Qaeda leaders.
-- Courts convicted 21 people in 2007 of the attacks. Four of the 21 had their convictions overturned in 2008.
-- Three weeks after the blasts, seven men including two suspected bombing ringleaders blew themselves up in an apartment after police closed in on them. The blast killed a policeman.
* London bombings:
-- Four suicide bomb blasts on London transport during the morning rush hour on July 7, 2005 killed 52 people and injured about 700 in the first Islamist suicide bombing attacks in western Europe. On July 21, four men made a failed attempt to carry out a second wave of attacks on three London underground stations and a bus.
* Barcelona plot:
-- Spain's high court on Dec 14, 2009 jailed 11 men for up to 14 and a half years for attempted suicide bombings on Barcelona's metro in 2008.
-- The group, including 10 Pakistanis and one Indian, were very close to developing explosives to be used in the attacks planned for Jan 18-20, 2008, according to the ruling. The group followed the "violent principles of jihad" of then Pakistani Taliban leader Baitullah Mehsud.
* Airline plot:
-- Three Britons were found guilty in September 2009 of plotting to kill thousands by blowing up transatlantic airliners bound for North America in mid-flight suicide attacks using bombs made from liquid explosives.
The bombers intended to simultaneously destroy at least seven planes carrying over 200 passengers each between London's Heathrow airport and the United States and Canada in August 2006 using explosives hidden in soft drink bottles, prosecutors said. The plot was hatched in Pakistan just months before the men were arrested in August 2006.
Police suspected al Qaeda planner, Egyptian Abu Obaidah al Masri, who some media reports have cited as the inspiration for the July 7, 2005 suicide bombings in London, was the mastermind.
* Northern England plot:
-- Pakistanis Abid Naseer and Ahmad Faraz Khan were among 12 men arrested in raids across northwest England in April 2009. Britain believed the men were part of a plot to carry out a "mass casualty" attack that month but there was not enough evidence to charge them and they were ordered to be deported.
Naseer and Khan won appeals against their deportation in May 2010 because of concern for their safety in Pakistan. But the Special Immigration Appeals Commission said it was satisfied Naseer was an al Qaeda operative.
* Shoe bomber:
-- Richard Reid, a British-born follower of Osama bin Laden, was sentenced to life in prison in January 2003 for trying to blow up a transatlantic flight with explosives stuffed in his shoes. Reid tried to blow up American Airlines Flight 63 on Dec 22, 2001, as it flew to Miami from Paris. He was unapologetic for his actions and said he was "at war" with the United States because it sponsored "rape and torture".