A massive blaze tore through a Bangladesh factory killing at least 52 people trapped by flames that forced workers to leap for their lives from upper floors, emergency services said on Friday.

About 30 people were injured in the fire, and hundreds of distraught relatives and other workers waited anxiously outside the food factory as it continued to rage.

The inferno was the latest to tarnish Bangladesh's safety record marred by a series of disasters in industrial complexes and apartment buildings.

The country has pledged reforms since the Rana Plaza disaster in 2013 when a nine-storey complex collapsed killing more than 1,100 people. But critics say safety standards are still lax. 

In February 2019 at least 70 people died when an inferno ripped through Dhaka apartments where chemicals were illegally stored.

The latest fire broke out at Hashem Food and Beverage factory in Rupganj, an industrial town outside Dhaka, on Thursday afternoon and was still raging almost 24 hours later.

- Rescued on ropes -

Normally there were would be more than 1,000 workers in the building but many had left for the day when the fire started.

The overnight toll of three dead rose dramatically as firefighters reached the third floor and found 49 more bodies.

Fire service spokesman Debashish Bardhan said: "The workers could not go to the rooftop because the exit door to the staircase was padlocked. They could not go down because the lower floors were already engulfed by fire."

The charred victims were piled into a fleet of ambulances to take them to mortuaries amid anguished shouts and tears from people watching in the streets.

Police dispersed hundreds of people who blocked nearby roads, while some clashed with officers. 

More than 30 people were injured and some jumped from the upper floors as flames engulfed the six-storey building, police inspector Sheikh Kabirul Islam said.

Emergency services were battling to put out the fire on the fifth and sixth floors. Firefighters using ropes rescued 25 people from the roof of the factory that made noodles, fruit juices and candy. 

"Once the fire is under control, we will conduct a search and rescue operation inside. Then we can confirm any further casualties," Bardhan told AFP. 

Dhaka fire chief Dinu Moni Sharma Sharma said the blaze took off because highly flammable chemicals and plastics had been stockpiled inside. 

Mohammad Saiful, a factory worker who escaped, said dozens of people were inside when the blaze erupted.

"On the third floor, gates on both stairwells were closed. Other colleagues are saying there were 48 people inside. I don't know what happened to them," he said.

Mamun, another worker, said he and 13 other workers ran to the roof after the fire broke out on the ground floor and black smoke quickly choked the whole factory.

He told how they had been brought down on ropes.

Other workers said there had been smaller fires in the building in recent years and that the factory had only two staircases for people to escape. 

As clouds of black smoke billowed from the building, many of the waiting relatives said they feared the worst.

Pakhi Begum, 50, said she had been to every local hospital and not found her son. "He is not there. I am afraid he is dead," she told AFP.       

Nazrul Islam said: "We came here because my niece was not answering our phone calls for a while. And now the phone is not ringing at all. We are worried."