Officials and witnesses said fireworks were used inside the club, while Colectiv Club`s Facebook page advertised pyrotechnic effects at the show.
Deputy Interior Minister Raed Arafat said 17 of the 27 dead had yet to be identified and 146 people remained in hospital. He said no fire permit was requested by the club nor granted to them by the Bucharest firefighting department.
"Unfortunately, the death toll may change taking into account the severity of their injuries," Arafat said after an emergency meeting early on Saturday.
President Klaus Iohannis toured Bucharest hospitals to visit the victims and also lit a candle at the club, while some 600 people queued to donate blood.
"I`ve got strong clues the law was broken in this case. I`m revolted that such a tragedy happens in downtown of the capital and innocent youngsters are perished," Iohannis said.
A pillar covered with foam panels and the club`s ceiling went up in flames, followed by an explosion and heavy smoke, the witnesses said. Many people admitted to 12 hospitals had suffered burn, smoke inhalation injuries or were trampled.
TV footage showed police officers and paramedics trying to resuscitate young people lying on the pavement while sirens wailed with more ambulances deployed to the scene.
Prime Minister Victor Ponta, just back from an official visit to Mexico, ordered checks on clubs across the country to see whether safety and firefighting norms are being observed.
"There was a stampede of people running out of the (Colectiv) club," a man who escaped without shoes told Reuters.
A young woman who was released from the hospital after minor injuries described the club bursting into flames.
"In five seconds the whole ceiling was all on fire. In the next three, we rushed to a single door," she told television station Antena 3.
Deputy Prime Minister Gabriel Oprea said a criminal investigation into the causes of the incident was already under way at the General Prosecutor`s office on suspicion of murder and destruction crimes, but no accusations were yet pressed.
Any open fire displays and fireworks in Romania require special authorization if used in a public indoor place. Such permits may be granted if the venue is assessed to be safe and equipped with extinguishers, and the fire department deploys several firefighters to the place.
European Commission President Jean-Claude Juncker sent a message of condolences: "I am greatly saddened to see so many young lives ending so tragically. My thoughts are with the grieving families and friends as well as with all those working hard in rescuing and in assisting the victims."
Some of the deadliest nightclub disasters in the world have been caused by fireworks.
In the southern Brazilian college town of Santa Maria in 2013, a musician lit an outdoor flare inside the Kiss nightclub and started a fire that killed at least 241 people.
Fireworks were also blamed for nightclub fires in Russia’s Perm that killed 156 people in 2009 and in Argentina’s Buenos Aires in 2004 that killed 194.
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