A five-storey building toppled onto three tin-shed houses in the capital's Begunbari area last night killing at least four people and trapping 20 others.
Scores are feared dead as about 200 people resided in the three houses with 50 rooms.
Army personnel joined the rescue operation shortly after 2:00am.
At least 25 people were injured in the incident. Of them, two were in a critical condition.
Fire-fighters rescued a child and pulled out four bodies of rubble. The dead are one-and-a-half years old Rabbi, his sister Jannat, 11, and Imon, 32, and his wife Kalpana, 20.
"At least 20 people remain missing," said Mohibul Haque, deputy commissioner of Dhaka, after visiting the spot.
Two bodies were brought to Dhaka Medical College Hospital Morgue, said staff at the morgue.
Fire-fighters could not start a full-scale rescue operation even three hours into the incident as the lanes around the building are too narrow for rescue vehicles to pass.
Leakage in gas pipes and snapped electricity cables made the rescue work risky, said the deputy commissioner.
Several dwellers of three nearby houses said they came out hearing a loud noise and saw the five-storey building falling down slowly on the three houses.
Most residents of the five-storey structure escaped unharmed while many dwellers of the tin-shed houses remained trapped under rubble.
Bharat Chandra Biswas, a deputy director of the fire brigade, said he saw a woman trapped under rubble but she was not seriously injured.
Four others along with the woman remained trapped under the debris, said Bharat, who was leading the rescue operation.
Locals said Abbasuddin, a retired driver of the General Post Office in Dhaka, owns the building and his son Dalim Mia maintained it.
It was a three-storey building until 2001 and later made to a five-storey structure. The construction of the top floor was completed just a weak ago, they said.
The building toppled onto three houses of Momtaz Begum, Abdul Zabbar and Monir Hossain.
The lanes around the site are so narrow that no rescue vehicle or ambulance could go within 200 metres of the spot.
Relatives of the missing people were seen wailing for their dear ones.
Alamgir Hossain, a worker of a phone company, said he returned home at about 10:30pm and found his rented house under rubble with his wife Anwara, 22, and children Shaon 8, Achhia 3 and Bishal.
Fire-fighters were struggling to conduct rescue work as the electricity connection got snapped in the incident.
A rescue worker said they were searching for survivors without any equipment.
Army members later rushed to the spot to help fire-fighters in the rescue work.
"We might engage more people in the rescue operation. But it is not possible to use any heavy equipment here," said Lt Colonel Iftekhar.