In some places, they were able to cut holes into the jumbled mess of concrete with drilling machines or their bare hands, and pass water and flashlights to people trapped inside.
"I gave them whistles, water, torchlights, I heard them cry. We can't leave them behind this way," said fire official Abul Khayer. He said rescue operations would continue through the night because many survivors were still believed to be inside the collapsed building.
Less than five months after a factory fire killed 112 people, the disaster Wednesday again underscored the unsafe conditions in Bangladesh's massive garment industry. Workers said they had hesitated to go to work because the building had developed such large cracks the previous day that it had been reported on local news channels.
Abdur Rahim, who worked on the fifth floor, said a factory manager assured them there was no problem, so they went inside.
"We started working. After about an hour or so the building collapsed suddenly," he said. He next remembered regaining consciousness outside the building.
Home Minister Muhiuddin Khan Alamgir told reporters during a visit to the site that the building had violated construction codes and "the culprits would be punished."
Among the textile businesses in the building were Phantom Apparels Ltd., New Wave Style Ltd., New Wave Bottoms Ltd. and New Wave Brothers Ltd., which make clothing for brands including The Children's Place and Dress Barn.
Jane Singer, a spokeswoman for The Children's Place, said that "while one of the garment factories located in the building complex has produced apparel for The Children's Place, none of our product was in production at the time of this accident."
"Our deepest sympathies go out to the victims of this terrible tragedy and their families," Singer said in a statement.
Dress Barn said it to its knowledge it had "not purchased any clothing from that facility since 2010. We work with suppliers around the world to manufacture our clothing, and have a supply chain transparency program to protect the rights of workers and their safety."
Workers said they didn't know what specific clothing brands were being produced in the building because labels are attached after the products are finished.
Sumi, a 25-year-old worker who goes by one name, said she was sewing jeans on the fifth floor with at least 400 others when the building fell. "It collapsed all of a sudden," she said. "No shaking, no indication. It just collapsed on us."
She said she managed to reach a hole in the building through which rescuers pulled her out.
Reports suggested the death toll was likely to rise.
"We sent two people inside the building and we could rescue at least 20 people alive. They also told us that at least 100 to 150 people are injured and about 50 dead people are still trapped inside this floor," said Mohammad Humayun, a supervisor at one of the garment factories.
Tens of thousands of people gathered at the site, some of them weeping survivors, some searching for family members. Firefighters and soldiers using drilling machines and cranes worked with local volunteers in the search for survivors.
An enormous section of the concrete structure appeared to have splintered like twigs. Colorful sheets of fabric were tied to upper floors of the wreckage so those inside could climb or slide down and escape.
Rescuers carried the body of a young boy from the building, but it was not immediately clear what he had been doing inside. The building, in the Dhaka suburb of Savar, housed a bank and various shops in addition to the garment factories.
An arm jutted out of one section of the rubble. The lifeless body of a woman covered in dust could be seen in another.
Rahim said his mother and father, who worked with him in the factory, were trapped inside.
"I have no idea what is going on," he said.
Mosammat Khurshida wailed as she looked for her husband. "He came to work in the morning. I can't find him," she said. "I don't know where he is. He does not pick up his phone."
Zahidur Rahman, a spokesman for Enam Medical College and Hospital, said by Wednesday evening 87 people had been confirmed dead. Brig. Gen. Mohammed Siddiqul Alam Shikder said 600 people had been rescued.
At the morgue of the medical college, many wailed as they waited for the bodies of their loved ones. "Where's my mother? Where's my mother? Tell me, tell me, oh Allah, oh Allah," Rana Ahmed cried.
The November fire at the Tazreen garment factory drew international attention to working conditions in Bangladesh's $20 billion-a-year textile industry. The country has about 4,000 garment factories and exports clothes to leading Western retailers. The industry wields vast power in the South Asian nation.
Tazreen lacked emergency exits and its owner said only three floors of the eight-story building were legally built. Surviving employees said gates had been locked and managers had told them to go back to work after the fire alarm went off.