Russian investigators have arrested the owner of a nightclub and several others over the deaths of 109 people in a fire, sparked by an indoor fireworks show.
Officials say the nightclub had ignored repeated warnings about its fire safety, the BBC reports.
Russian President Dmitry Medvedev said the fire was a criminal act and those responsible must be punished. He has declared a national day of mourning on 7 December.
Mr Medvedev said those responsible "have neither brains, nor conscience", calling for the toughest possible punishment.
Regional public security minister Igor Orlov says one of the fireworks hit the club's plastic ceiling and started a fire. Panic ensued, he says, and many of those trying to flee succumbed to burns, crushing and gas poisoning.
The deaths occurred at the packed Lame Horse nightclub in the city of Perm after fireworks being used in a stage show sparked an explosive blaze.
A witness says dozens of bodies were piled on the pavement outside the club. More than 140 people were reported injured.
Any connection to train bombing dismissed
The authorities in Russia have been quick to say that the deaths were the result of an accident and not a terrorist attack.
The fire follows a bombing last week that killed 26 and injured more than 100 on a train travelling between Moscow and St Petersburg, for which Chechen Islamist militants claimed responsibility.
That aroused fears that Russia might be in for a nationwide bombing campaign, but officials are playing down any links between the train attack and the nightclub disaster.
Friends, relatives of owner were celebrating
Most of those in the club when the fire started late on Friday night were friends or relatives of the owner, celebrating the club's eighth anniversary.
Regional officials told the Itar-Tass news agency that the fireworks were intended for outdoor use and not for closed premises, the BBC reports.
Fireworks are considered to have been one of the causes of a Bangkok nightclub fire in January that killed 65 people.
One of Russia's biggest cities, with a population of 1.2 million, Perm is about 1400km east of Moscow.