Dhaka, Bangladesh (CNN) -- An eight-story building collapsed Wednesday on the outskirts of the Bangladeshi capital, killing at least 123 people and injuring more than 1,000, officials said.
Broken concrete and twisted metal stuck out from the massive pile of rubble, where rescuers dug by hand to reach trapped workers. Dhaka District Police Chief Habibur Rahman said search efforts were continuing Wednesday night. It was unclear how many people were trapped in the ruins, he said, and the death toll could increase.
Video from the scene showed injured victims being whisked away on stretchers and crews carrying limp bodies from the rubble. Some onlookers wept while others frantically searched for missing loved ones.
Abdul Alim said he heard people screaming inside the building and tried to help them.
"But ... we couldn't make our way to get in," said Alim, a day laborer who was one of the thousands of onlookers who tried to reach trapped victims before military, fire and civil defense rescuers arrived at the scene.
Authorities said they had not yet determined what caused the collapse of the building, where five garment manufacturers employing about 2,500 workers were based, according to the Bangladesh Garments Manufacturers and Exporters Association.
Several garment workers near the wreckage said a crack appeared Tuesday on the building's seventh floor.
At first, the workers said, managers ordered workers not to report to work on Wednesday.
Later, the factory owners reversed the order, telling workers that the building was safe, said Marjina Begum, who worked on the sixth floor. Many workers were hesitant to show up Wednesday but reported to work because they were afraid of losing their jobs, she said. More than a dozen other workers corroborated her story.
Managers for the garment manufacturers housed inside the building could not be immediately reached for comment.
M. Atiqul Islam, president of the garment manufacturers association, told reporters Wednesday evening that owners kept the factories open after the building's owner assured them that there would be no problems with the cracks that appeared on Tuesday.
Employees of the Savar branch of BRAC Bank were evacuated Tuesday after the crack was detected and were ordered not to show up on Wednesday, according to the national news agency, Bangladesh Sangbad Sangstha, citing a news statement from the bank.
None of the bank's workers was among the injured or dead, it said.
In addition, a general strike had shut a shopping mall on the two lowest floors.
The third floor of the building was offices, garment manufacturers were on the fourth through seventh floors, and the top floor was a canteen.
Bangladesh Home Minister Muhiuddin Khan Alamgir said military troops, fire service personnel and police had been mobilized to the site, about 25 kilometers (15 1/2 miles) from Dhaka.
"The fact is, we don't know yet how many people were killed actually ... but I can tell you the building was not built in compliance with the (safety) rules and regulations," Alamgir said.
"Stern legal actions will be taken against the people who built the structure defying the codes or laws."
Local police said the building's owner had gone into hiding.
A neighboring three-story building also collapsed as part of the eight-story building fell on it, police said.
Rescue work was proceeding slowly to avoid causing a further collapse, said Maj. Gen. Abul Hassan Sarwardy, commander of the army's Savar-based 9th Division.
Hospitals in Savar and Dhaka treated more than 1,000 people who were rescued from the collapsed buildings, officials said.
Video showed two female garment workers pleading to be rescued. "Please retrieve us. ... Save my life," said a woman who identified herself as Sakhina.
A doctor at Enam Hospital said some victims had lost their hands, arms or legs.
A national day of mourning was set for Thursday.
The building housed several garment makers -- including New Wave Style, Ether Tex, Canton Tech Apparel and New Wave Bottoms. The contractors appeal to cost-conscious merchants because of the low wages they can pay Bangladeshi workers.
It was not immediately clear which retailers were doing business with the factories.
A supplier to British retailer Primark was located on the second floor of the building.
"The company is shocked and deeply saddened by this appalling incident at Savar, near Dhaka, and expresses its condolences to all of those involved," Primark said in a statement.
"Primark has been engaged for several years with NGOs and other retailers to review the Bangladeshi industry's approach to factory standards," it said. "Primark will push for this review to also include building integrity."
The last major building collapse in the country occurred in 2005, when more than 70 people were killed in a garment factory collapse in the same area, the Bangladesh Sangbad Sangstha news service reported.
A fire at a clothing factory in a suburb of Dhaka in November killed at least 112 people.
Garments account for 80% of Bangladesh's $24 billion of exports.
The country has about 4,500 garment factories where workers make clothes for brands including Tesco, Walmart, JC Penney, Kohl's and Carrefour.
Scott Nova, executive director of the Worker Rights Consortium, said the disasters keep happening because companies put pressure on factories to lower prices, which results in substandard safety conditions.
"The worse the dangers get, the more business comes in, so the government has no incentive to fix anything," Nova said. "We ask ourselves every day what it's going to take to fix this."
CNN's Samira Said and Sumnima Udas and CNNMoney's Emily Jane Fox contributed to this report.