Masked gunmen robbed a post office in Gothenburg early Tuesday, leaving behind burning cars and suspected bombs that caused chaos in Sweden's second largest city.

The robbers struck the busy Krusthusgatan post office in the middle of the night but it was not immediately clear what they escaped with.

"An unknown number of masked, armed men threatened employees and carried out a robbery," Gothenburg police said in a statement.

The robbers fled about 15 minutes later, setting several cars in the city centre on fire and leaving behind a number of "suspicious objects", feared to be bombs, police said.

One suspected bomb was left in front of police headquarters and a number of police officers were reportedly stuck inside.

Some 120 people were evacuated from a nearby building, and large parts of central Gothenburg remained blocked off on Tuesday morning, police said.

Per Ljungberg, a spokesperson for the Swedish postal service, said the Gothenburg post terminal was the largest in the region and handled about two million letters a day.

"We're a bit surprised by the robbery. There are obviously some valuable packages that pass through, but it's a bit like looking for a needle in a haystack," he told AFP.

He said none of the dozen or so employees present at the time of the heist had been injured, "but many are in shock".

Former US vice president and Nobel Peace Prize laureate Al Gore was expected in the city later Tuesday to receive the Gothenburg Award for Sustainable Development at a conference on climate change.

(english text source: http://australianetwork.com/news/stories/asiapacific_stories_2144218.htm)

note: i put wrong time to the video. the police warned the inhabitants 04:30, not hour earlier as i wrote on the video.