A fire Tuesday at a popular Istanbul nightclub that was undergoing renovations killed 29 people and injured one, the governor’s office said.

The authorities said the renovation work at the club, Masquerade, may have caused the fire, which took place while the club was closed to the public. The dead and injured included construction workers, the governor of Istanbul, Davut Gul, said in televised remarks.

The police arrested six people in connection with the fire, including two partners of the club, two managers, a person who was in charge of renovation work and an accountant, the governor’s office said, but no charges were immediately disclosed. The justice ministry assigned three prosecutors to investigate the case.

The nightclub is on the two basement floors of a 16-story residential building in downtown Istanbul, raising questions about whether it was following fire regulations including those requiring proper evacuation routes and the use of fire-retardant material.

The nightclub management did not have a permit to conduct the renovations, the mayor of Istanbul, Ekrem Imamoglu, said in televised remarks. Inside the club, construction material was heaped together “in a very irregular manner,” Mr. Imamoglu said, making the rescue work harder for firefighters.

Firefighters carried at least three people from the scene on stretchers, two of them covered with black and gray smoke, according to footage from the Demiroren news agency.

The nightclub has long been a popular site in Istanbul, hosting concerts of hip-hop and R&B stars like 50 Cent, Tyga and Ciara.