The AquaDom aquarium is located in the foyer of a Radisson Collection hotel, in the DomAquarée business and leisure complex.
"In addition to the unbelievable maritime damage (...) two people were injured by glass splinters," Berlin police said on Twitter.
The city's fire service tweeted that 100 firefighters had been deployed to the scene. ''The aquarium is damaged, water is leaking'', the tweet said, adding ''the situation is not clear at the moment.''
Berlin mayor Franziska Giffey said on Twitter that around 1 million liters (264,000 gallons) of water had leaked from the AquaDom.
Fire department spokesman Adrian Wentzel told CNN affiliate NTV that the incident happened around 5:50 a.m. local time. Wentzel said glass and other debris were swept out of the hotel onto the nearby street. Emergency services were forced to shut a major road next to the building because of the large volume of water flooding out. Wentzel said the cause of the incident was still being investigated.
Footage shared by hotel guests on social media on Friday shows extensive damage to the giant structure, with glass and debris covering the floor.
"Just chaos. The whole aquarium burst and what's left is total devastation. Lots of dead fish, debris. It is quite a drama," a hotel guest told Reuters.
Radisson Hotels said on its website that guests would be relocated and the hotel was closed until further notice.
According to the DomAquarée complex website, AquaDom was the world's largest freestanding cylindrical aquarium, and it contained a clear-walled elevator for visitors. The complex also houses apartments, museums, shops, cafes and restaurants. The aquarium was last modernized in 2020, the operator said.
Later Friday, AquaDom's owner, real estate investment company Union Investment, issued a statement.
"We are dismayed at the accident and wish the two injured persons all the best and a speedy recovery. We would also like to express our dismay to our affected tenants," the statement read.
The company added that "a large portion" of the aquarium's fish died due to the "severe destruction of the acrylic cylinder and the leakage of water" and that rescue operations are underway to save the others.
"We are currently trying to get a more precise picture of the situation and the damage caused in coordination with the police and fire brigade on site," it said.
Nadine Schmidt, CNN