Twenty-seven people - mostly teenagers and young people - have been killed after fire broke out at a nightclub in Bucharest, officials say, reports BBC.The blaze took hold at the Colectiv club on Friday night, causing a stampede for the exit.Emergency response chief Raed Arafat said 155 people were being treated in hospitals in the Romanian capital.The fire is believed to have been caused by fireworks that were let off inside the club."The only information we have is that fireworks were used in the club and after that the tragedy happened. Of course, this is under investigation," Mr Arafat told the BBC.The pyrotechnics were reportedly part of a show by a heavy metal band.Witnesses said a spark on the stage ignited some of the polystyrene decor. A pillar and the club`s ceiling caught fire and there was an explosion and heavy smoke, they added."People were fainting, they were fainting from the smoke. It was total chaos, people were trampling on each other," Victor Ionescu, who was at the club, told local TV station Antena 3.Up to 400 people may have been inside the club, which was hosting a free rock concert.Local journalist Sorin Bogdan told the BBC the club was in a converted former factory with two small exit doors, only one of which was possible to open initially.A witness quoted by Romania`s state news agency said terrified concert-goers had to break the second door down to escape.Many of those being treated in hospital were suffering from smoke inhalation and severe burns, Mr Arafat said.At least 25 people were reported to be in serious condition at the Municipal Hospital.Staff at another hospital said most of the people they treated were teenagers aged between 14 and 16.Prime Minister Victor Ponta said he was cutting short a visit to Mexico to return to Bucharest.Romania`s President Klaus Iohannis wrote on his Facebook page that he was "deeply grieved by the tragic events that happened this evening in downtown."He added: "It is a very sad day for all of us, for our nation and for me personally."