At least 109 people have died following an explosion at a nightclub in the Russian city of Perm, 1,400km (870 miles) east of Moscow, BBC reported.
Officials said fireworks caused the blast and that most victims had died from smoke inhalation.
More than 140 people were reported injured in the accident, which happened at 2315 local time (1815 GMT).
The Lame Horse nightclub had been celebrating its eighth anniversary, emergency services said.
Regional public security minister Igor Orlov told Itar-Tass news agency: "There were fireworks launched at the scene, and one hit the plastic ceiling, setting all ablaze. People panicked and succumbed to burns, general crush and gas poisoning."
Russian state-run TV channel Vesti-24 showed bodies piled in the street outside the club.
Perm has a population of 1.2 million and is the sixth largest city in Russia.
Svetlana Kuvshinova, who was in the club when the fire began, said the flames "took seconds to spread".
"It was like a dry haystack. There was only one way out. They nearly stampeded me," the Associated Press quoted her as saying.
Another eyewitness told local TV: "When the merry-making started, when they were warming up, they set off fireworks.
"When I turned back, I saw drops falling off the ceiling and then there was a lot of smoke. Later this huge flame was moving along the ceiling," the unnamed woman said.
A spokesman for the prosecutor-general's main investigative unit told Itar-Tass that it was not a terrorist attack.
"We are talking about a failure to observe fire regulations," he said.
The Russian government has set up a special commission to deal with the incident.
Regional officials said that fireworks used at the party were too big for closed premises and were intended for outdoor use.
Dmitry Peskov, spokesman for Russian Prime Minister Vladimir Putin, said two planes equipped to transport people with serious burns were being sent to Perm.
Government officials were also leaving for the city, he said.
The BBC's Tom Esslemont, in Moscow, says Russia has a very poor fire safety record with hundreds of people dying in similar incidents every year.