Bangladesh police on Saturday arrested for murder the owner of a factory where 52 people died in an inferno, as it emerged that children as young as 11 had been working there.

Police said the owner of the food factory and four of his sons were among eight people detained over the fire that broke out Thursday and raged for more than a day.

A separate inquiry has been launched into the use of child labour at the facility.

Jayedul Alam, police chief for Narayanganj district where the factory is located, said the entrance had been padlocked at the time of the blaze and the factory breached multiple fire and safety regulations.

"It was a deliberate murder,” the police chief told AFP.

Emergency services found 48 of the bodies on the third floor of the Hashem Food and Beverage factory in Rupganj, an industrial town outside Dhaka.

A fire services spokesman said the exit door to the main staircase had been padlocked. Highly flammable chemicals and plastics had also been stored in the building.

Monnujan Sufian, state minister for labour, said inquiries had begun into the use of child workers at the factory.

Sufian told AFP she had spoken at a hospital to two survivors aged 14. One woman said her 11-year-old nephew had been working at the factory and was missing, feared dead.

Bangladesh pledged reforms after the Rana Plaza disaster in 2013 when a nine-storey complex collapsed killing more than 1,100 people.

But there have been a series of fires and other disasters since then. In February 2019 at least 70 people died when a fire ripped through Dhaka apartments where chemicals were illegally stored.