The explosion occurred late on Friday at a venue called Colectiv Club, where some 400 people, including children, were inside the building at the time of the incident, Reuters reported.
According to one witness speaking to Romanian media, a fireworks display around the stage set nearby objects alight.
Another witness told media that a pillar and the club’s ceiling caught fire, then there was an explosion and heavy smoke.
Some of the seriously wounded people were taken to nearby hospitals while those with milder injuries were treated on pavements outside of the building.
Romanian President Klaus Iohannis also expressed his sympathy and solidarity with the families of the victims.
“I am shocked and deeply saddened by the tragic event that occurred tonight. It is a very sad moment for our nation,” Iohannis wrote on his Facebook page.