Fire at Bucharest nightclub
Emergency services treat the wounded following a fire at a Bucharest nightclub Reuters

Twenty-seven people – mainly teenagers and young people – have died following a fire in a Bucharest nightclub believed to have been caused by a firework going off during a heavy metal band's free Halloween gig.

At first people thought the pyrotechnics were part of Goodbye to Gravity's act at the Colectiv club at around 10.30pm on 30 October. A mass stampede for the exit followed when revellers realised the club was on fire.

Raed Arafat, secretary of state for the interior, said 155 people were being treated in hospitals in the Romanian capital. Nine are reported to be in a critical condition. "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," Arafat told the BBC.

Band Goodbye to Gravity were hosting a free rock concert for Halloween in front of an audience of up to 400 people. Witnesses said that a spark on the stage ignited polystyrene cladding around a pillar. The flame spread to the club's ceiling and there was an explosion, accompanied by heavy smoke.

"People were fainting, they were fainting from the smoke. It was total chaos, people were trampling on each other," Victor Ionescu, who was inside the club, said to Bucharest TV station Antena 3. Audience members initially thought the flames were part of the show and did not react immediately, according to local reporting.

At least 25 people are reported to be in serious condition at the city's Municipal Hospital. Medical staff at another hospital in the capital said the majority of the victims were teenagers aged between 14 and 16. The band's singer and bassist are now believed to be in a serious condition. Many of those being treated in hospital were suffering from smoke inhalation and severe burns, said Arafat.

There has been criticism of lax fire regulations in Bucharest clubs and restaurants. Bucharest journalist Sorin Bogdan said the club was housed in a converted factory with only two small exit doors, only one of which was opened initially. A witness told Romania's state news agency concertgoers had to break down the second door.

Romanian President Klaus Iohannis wrote on his Facebook page to say that he was "shaken and deeply hurt by the tragic event. ... It is a very sad moment for us all, for our nation and for me personally". Meanwhile, Prime Minister Victor Ponta said he was terminating his trip to Mexico to return to Bucharest. On 31 October Romanian officials announced three days of national mourning.