STOCKHOLM - Sweden's prime minister on Sunday urged the country to "stand up for tolerance" after a botched terrorist attack in central Stockholm highlighted signs of growing Islamic extremism across the usually peaceful Scandinavian region.

Fredrik Reinfeldt condemned Saturday's attack, in which a suspected suicide bomber was killed, as an assault on Sweden's open society that risked inflaming racial tensions in a country with a large Muslim population.

The incident followed recent warnings from Sweden's security service of an increased threat from Islamic extremists and came amid a Norwegian investigation into a suspected terrorist plot aimed at neighboring Denmark.

All three countries are grappling with how to tackle radicalization among parts of their growing immigrant populations, while preserving the region's traditional social liberalism.

Swedish security forces Sunday were racing to establish whether the Stockholm attack, which targeted a busy pedestrian thoroughfare packed with Christmas shoppers, was the work of a "lone wolf" or part of a broader plot.

Reinfeldt said it was too soon to know whether the man killed in the apparent suicide explosion also was responsible for a car bomb that detonated nearby at about the same time, injuring two passersby.

Local media reports said police believed they had identified the suspected bomber as a 29-year-old immigrant from southern Sweden, whose name was connected with the white Audi involved in the blast.

Neither the car bomb nor the suspected suicide bomb caused significant damage beyond the immediate vicinity, suggesting they might not have detonated properly. Reports said unexploded pipe bombs and a bag of nails were found near the dead man's body. Experts said only luck - and possibly poor bomb-making - prevented further loss of life.

A threatening message was sent to Swedish media shortly before the explosions, warning of punishment for the presence of Swedish forces in Afghanistan and for the publication of caricatures of the prophet Muhammad by a Swedish cartoonist.

Police said they could not confirm that the e-mail message, which called for an uprising by mujahedin fighters across Europe, was connected with the attack.

Security also has been heightened in Germany and other European countries in recent weeks, after warnings that Islamic terrorists could be planning an assault.

The Stockholm excplosions were the latest in a series of botched attacks by Islamic extremists in Western countries, including a failed car bomb in New York City's Times Square in May and a similar incident at the Glasgow airport in Scotland in 2007.