The giant glass roof of a swimming pool has collapsed at a Moscow water park, killing at least 15 people and injuring nearly 100.
Hundreds of people were in the building at the time
It is thought that poor construction or heavy snow piled up on the roof may be responsible for the incident at the Transvaal Park in south-west Moscow.
Russia's Emergency Ministry said three children were among the dead.
Terrified visitors rushed out into the icy cold in their swimming costumes, as others remained trapped in the debris.
Dozens of people - some with serious injuries - were rushed to hospitals amid fears that the death toll would rise further, emergency services said.
Rescuers continued their search for survivors through the night, stopping at regular intervals to listen for voices in the rubble.
"Every half an hour, we have a minute of silence so that we can hear where the voices are coming from," a rescue worker told the Russian Itar-Tass news agency.
Emergency services have been pumping hot air into the wreckage to prevent the pool freezing over possible survivors.
A Moscow district prosecutor has said he is ordering an investigation to see if local government officials and the owners of the sports park should be held responsible.
Russian officials dismissed initial reports that an explosion might have occurred at the leisure complex, situated in the Yasenevo district.
"It has been definitely confirmed it was not an act of terror. The moment the roof caved in was recorded by video cameras," Moscow Mayor Yuri Luzhkov told reporters outside the complex.
The building had huge glass windows
The mayor also told the BBC Russian Service that there was no snow on the roof.
Dozens of ambulances rushed to the scene, along with several fire engines.
Rescue teams continued to dig up quickly as those who may be still inside the complex did not have much chance to survive in freezing temperatures of up to -15C (5F) .
The Emergencies Minister, Sergei Shoigu, also went to the pool.
More than 420 people were in the area where the roof caved at about 1930 local time (1630 GMT), Mayor Luzhkov said.
He said that up to 1,300 visitors were in the complex at the time of the incident.
The total area of the collapsed roof was 5,000 square metres.
Russian television footage from inside the park showed that beams - apparently made of concrete - had collapsed onto the pool's twisting water slide.
"Everything in blood'
Witnesses spoke of their horror and panic when they heard the cracking sound of the roof caving in.
"There was a loud noise," one of the witnesses told Russia's Channel One television.
"Everybody started to run. I started to run. All of the roof over the water zone collapsed."
Another witness, Roman Yazymin, told the Associated Press news agency that he was sun-tanning in a solarium on the upper floors of the complex when he heard a loud noise and the crash of shattering glass.
"Everything was in blood," he said as he walked through the complex to retrieve his clothing.
Transvaal, which opened in 2002, is Moscow's biggest water park.
In addition to a large pool, the park has an artificial river, a bowling alley and saunas.
Moscow has been extra vigilant since a bomb blast on the underground railway on 6 February, which killed 41 people and wounded more than 100. Officials blamed Chechen rebels for the attack.