A huge aquarium in Berlin exploded on Friday morning causing a million litres of water to flood a hotel and burst out onto the street, police said.

Glass and other debris were swept out of the building -- which also contains a hotel and cafes -- after the structure exploded around 6 am CET.

German police said two people were injured by glass shards and that the explosion had caused "incredible maritime damage". 

The 25-metre-high aquarium -- which contained 1,500 fish, all of whom died -- was housed at the Raddison Collection Hotel. 

"This morning at approximately 6.30 am CET the aquarium of the Aquadom Berlin which is located in the Radisson Collection Hotel, Berlin collapsed," a spokeswoman for the hotel told Euronews in a statement.

"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."

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The aquarium is the “largest, cylindrical, free-standing aquarium in the world,” according to Sea Life Berlin. According to the website, “Around 1,500 fish from over 100 different species live in the tropical tank.”

There was speculation that freezing temperatures had contributed to the leak, but Stralau said the cause of the incident was still being investigated.

Witness Gwendolin Szyszkowitz told German news channel n-tv that she heard a loud bang and initially feared a bomb had exploded.

Mayor Franziska Giffey said the explosion had unleashed a “veritable tsunami” of water but the early morning timing had prevented far more injuries.

“Despite all the destruction, we were still very lucky,” she said. “We would have had terrible human damage” had the aquarium burst even an hour later, once more people were awake and in the hotel and the surrounding area, she said.

The 10-minute elevator ride through the tank was one of the highlights of the attraction.

The AquaDom was opened in 2003 at a cost of €12.8 million. Guinness World Records said it was the "largest cylindrical aquarium".