"At least seven buildings have been engulfed by the fire. There were shops selling chemicals on the ground floor, which were caught by the fire as it spread very quickly," he told the AFP news agency.
"The temperature and fumes became unbearable because of the chemicals," he said, adding that a bakery with several large gas burners had also caught fire.
"We struggled to get inside due to the narrow stairways of the very old buildings, it is almost impossible for us to get fire-fighting equipment into the area."
Hundreds of people were trapped in the burning buildings for hours as rescue workers struggled to contain the blaze.
Firefighters were hampered by the narrow lanes of the Kayettuli district , one of Dhaka's most densely populated areas.
AHM Ruhal Haque, Bangladesh's health minister, said that doctors from the Dhaka Medical College were called from their homes to treat an overwhelming number of patients who arrived at hospital in critical condition.
Shahidul Bari, a specialist at the burns unit, said that at least 100 people had been admitted with severe burns and smoke inhalation.
"Our unit is full and more patients are pouring in. It's a disaster of huge proportions," he said.
"Patients are being treated in the corridors and still more are coming in. We are sending the most critically ill to the hospital's emergency units."
Fires due to short-circuits, substandard wiring and electrical faults are common in Bangladesh, where building regulations are rarely enforced.