STOCKHOLM (Reuters) - Police were investigating an explosion outside an up-market shopping center in central Stockholm on Tuesday, although there were no immediate reports of injuries.

Police said they were called to the Mood shopping center around 1730 GMT after witnesses described hearing a loud blast and seeing a white flash.

"We have a damaged facade and a ruptured car window but we are not aware of any injuries to people. Obviously something has exploded but it doesn't appear to be major damage," said police spokeswoman Carina Skagerlind.

Sweden's security police raised the threat level to the second-highest level after the Paris attacks last year.

Before Christmas 2010 two bombs exploded in central Stockholm on busy shopping streets.

(Reporting by Johan Ahlander; Editing by Janet Lawrence)