"There was a small problem, a simple crack, and the water leaked," a police official said, declining to be identified.

One of the largest tanks in the world, the aquarium features the world's largest viewing panel at 32.8 meters (107-ft, 7-in) wide and 8.3 meters high.

It was designed to hold more than 33,000 living animals, representing more than 85 species including over 400 sharks and rays combined, according to Dubai Mall's website.

A Reuters witness said the mall, operated by Emaar Properties, had been partly evacuated and dozens of emergency vehicles were outside. Six divers entered the tank and appeared to be coordinating with workers outside the glass.

Laborers mopped up water from the floor.

Emaar's chairman Mohammed Alabbar denied there was a leak in the aquarium, saying there was a "technical fault in the operating device,” according to a statement carried on the country's official news agency WAM.

But a witness said water had been leaking from a crack in the aquarium glass.

"I saw a small crack in the aquarium glass and there was a little water coming out and a lot of water on the floor," said Ranjin, a 27-year old corporate secretary.

"The police came and evacuated the area around the aquarium."

Emaar Properties, the Arab world's largest developer, is also the builder of the Burj Khalifa, the world's tallest tower that early opened last month. It has since been closed to the public following a problem with the elevators used to ferry visitors to its 124th-story observation deck.