The death toll may still climb at the club, which hosted a rock concert attended by up to 400, Deputy Interior Minister Raed Arafat said.
One witness said there were fireworks inside the club. A pillar and the club's ceiling caught fire and then there was an explosion and heavy smoke.
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.
"There was a stampede of people running out of the (Colectiv) club," a man who escaped without shoes told Reuters.
Deputy Prime Minister Gabriel Oprea said an investigation into the causes of the incident was already underway.