At least 87 people have been killed and many others trapped after an eight-storey building housing garment factories collapsed outside the Bangladeshi capital, Dhaka.
Firefighters and army personnel are leading the operation to rescue those caught beneath the debris in Savar.
More than 1,000 people were injured. One official put the death toll at 127.
Cracks had been found in the building prior to the collapse, but owners told workers not to worry.
Building collapses are common in Bangladesh. Speaking at the scene, Home Minister Muhiuddin Khan Alamgir said the building had violated construction codes and "the culprits would be punished".
Bangladesh has one of the largest garment industries in the world, providing competitively priced clothes for major Western retailers which benefit from its widespread low-cost labour.
Tessel Pauli, a spokeswoman for the Amsterdam-based Clean Clothes Campaign, said activists at the scene had identified labels from European and US high-street brands.
"Immediate relief and long-term compensation must be provided by the brands who were sourcing from these factories, and responsibility taken for their lack of action to prevent this happening," she said in a statement to the BBC.
Primark, a clothes retailer with a large presence in Britain, confirmed that one of its suppliers was on the second floor of the Rana Plaza. It said it was "shocked and deeply saddened by the appalling incident" and that it would work with other retailers to review standards.
The Rana Plaza building contained several clothing factories, a bank and a market.
It collapsed at about 09:00 local time (03:00 GMT), during the morning rush hour.
It is not yet clear what caused the collapse, but local media reports said severe cracks were detected in the block on Tuesday.
One man rescued from the building told the BBC that factory owners had told workers on Wednesday morning "not to worry" and that "they said they had examined the crack".
Police told local media that the rear of the building had suddenly started to collapse on Wednesday morning, and within a short time the whole structure - except the main pillar and parts of the front wall - had caved-in, triggering panic.
An eyewitness described the moment of the building's collapse: "It became completely dark on this side. There was a lot of dust from the collapsing debris, so we ran downstairs. When we came out we saw the whole building collapsed."
Only the ground floor of the building remained intact after the collapse, officials said.
Sohel Rana, a local who rescued several people, told Bangladeshi newspaper The Daily Star that he had heard cries for help coming from under the rubble.
The scene looked like a "war zone", Dhaka resident Tahsin Mahmoo told the BBC, adding that appeals had been put out for citizens to donate blood.
Hundreds of people, anxious for news of friends and relatives, have gathered at the scene. Others are moving rubble using their bare hands.
"Already we've rescued three to four hundred people... Now we are cutting through the concrete walls and trying to get inside with the help of sniffer dogs," fire brigade chief Ali Ahmed Khan told the BBC Bengali service.
Prime Minister Sheikh Hasina has announced a national day of mourning on Thursday in memory of the victims.
In November, a fire at a garment factory in a Dhaka suburb drew international attention to working conditions in Bangladesh's textile industry.
At least 110 people died, triggering a public outcry about safety standards.
Western retail chains that buy from factories in Bangladesh urged factory owners to improve safety standards.
The last major building collapse was in 2010, when a four-storey building in Dhaka caved in, killing at least 25 people and injuring several others.
In 2005, there was a building collapse near the site of Wednesday's incident, killing 64 people.