But a fire that may have been sparked by pyrotechnics late Friday quickly erupted with dark, blinding smoke -- killing at least 27 people and injuring more than 160 others, authorities said.
The blaze left the 300 or 400 people packed inside scrambling toward the lone exit in one of Romania's deadliest fires in recent memory.
Eyewitnesses said many in the crowd believed the first moments of the fire were part of the show.
The government declared three days of mourning, and President Klaus Iohannis visited the hospital where many of the injured were taken.
The President left flowers outside the club, according to Romanian TV's Antena 3.
"I wanted to see the scene of the tragedy," the station quoted him as saying. "It's incredible. This location is completely unfit. ... It is unimaginable that this location hosted so many people at the concert. And it's unimaginable that this tragedy happened so quickly because simple rules were ignored."
Dr. Raed Arafat, head of the country's department of emergency situations, told CNN that fire officials had not issued any permits for a gathering of that size or for the use of fireworks.
Video from the scene showed first responders treating -- and in some cases attempting to revive -- the wounded on the street.
Authorities said the cause of the fire hasn't been determined.
Arafat said many victims suffered smoke inhalation and some had severe burns up to 80% of their bodies.
Many of the injured were in critical condition at 12 hospitals, authorities said. And as many as 17 of the dead had still not been identified because of the severity of their burns.
Arafat said hospitals were overwhelmed by the number of victims, and equipment from nonemergency hospitals had to be transferred to facilities treating the injured.
A prosecutor and fire specialists will investigate the scene and determine how the fire started, said Monica Dajbog, a spokeswoman for the Interior Ministry.
Paul Angelescu, a reporter with ProTV, said the mayor told him the club had all the necessary permits to operate as a club.
Iohannis said he was "shocked and deeply saddened" by the tragedy.
"It is a very sad moment for all of us, for our nation and for me personally," he said in a post to his Facebook page.
The fire appears to be the deadliest nightclub blaze since a 2013 fire
at a nightclub in Santa Maria, Brazil, killed more than 230 people.