Two people were injured by flying glass, according to the emergency services, as the 25m-high tank exploded.

Hundreds of hotel guests had to be evacuated.

Guests staying at the Radisson hotel heard a loud noise at 5.45am that sounded like a bomb, one witness told n-tv, a local news channel.

More than 100 police and fire brigade workers were at the scene.

The attraction, called the Aquadom, was the world's biggest freestanding cylindrical tank.

Water flooded from the complex, which also holds a museum, shops and cafes, into Karl Liebknecht Strasse, a major road, which was closed after the incident.

The road and pavements outside the complex were littered with debris.

"All 400 guests who were staying at the Radisson hotel have been evacuated," DW correspondent Anna Saraste tweeted on Friday morning.

"They are waiting to be transported to another hotel in Berlin."

Emergency services provided warm spaces for the guests as temperatures in the German capital on Friday morning were around -7C (19.4F).

The aquarium is a major tourist magnet in Berlin.

Visitors can take a 10-minute elevator ride through the tank, which is one of the highlights of the attraction.

Police refused to comment on what had caused the tank to burst, but there was local speculation that freezing temperatures had contributed to the incident.

The Radisson said: "The aquarium of the Aquadom Berlin which is located in the Radisson Collection hotel, Berlin, collapsed.

"As far as we know to date, no guests were injured and together with the authorities we are looking into the cause of the incident.

"We have immediately closed the hotel until further notice and are relocating guests."