Survivors cried out to rescuers Thursday from the rubble of a block of garment factories in Bangladesh that collapsed killing 175 people, sparking criticism of their Western clients.

As the plaintive appeals from the site of Bangladesh's worst industrial disaster filtered through the concrete, relatives desperate for news of their loved ones descended on the scene clutching their photographs.

Flags flew at half-mast and a day of national mourning was declared after the latest tragedy to strike Bangladesh's garment industry, a key driver of the impoverished nation's economy that has a shocking safety record.

More than 1,000 people were injured when the eight-storey building, housing five garment factories on the outskirts of Dhaka, imploded on Wednesday, after managers allegedly ignored workers' warnings that the building had become unstable.

"The death toll is now 175," said Wali Asraf, a senior police officer in the disaster control room.

Authorities say they do not know how many people are still trapped under the mountain of concrete, but firefighters said they could still hear desperate cries for help from multiple places.

"I just heard someone saying 'please save me'," said Mamun Mahmud, a senior fire-fighting official, adding that about 1,500 people had so far been rescued alive.

Mahbubur Rahman, the operations director of the fire service, told AFP that rescuers had stopped using heavy clearing equipment to avoid jeopardising the chances of survival for those trapped alive.

"We're digging carefully and using only small machines to cut through the pancaked floors," he said.

Body after body was laid out on the ground of a nearby school as thousands of people filed past them to find their missing relatives.

"I've seen all the bodies. My sister was not among them. She is also not in any of the hospitals," said Mukta Begum, holding the photo of her younger sibling Suryaban, a garment worker.

The accident has again highlighted safety problems and poor working conditions that plague the textile industry in Bangladesh, the world's second-biggest clothing exporter.

Last November a blaze at a factory making clothing for Walmart and other Western labels in Dhaka left 111 people dead, with survivors describing how fire exits were kept locked by site managers.

Tessel Pauli, a spokeswoman for the Amsterdam-based Clean Clothes Campaign, said the latest disaster was "symptomatic" of problems in Bangladesh, where foreign buyers are accused of jeopardising safety in their search for profit.

"These accidents represent a failure of these brands to make safety a priority. They know what needs to be done and they are not doing it," Pauli told AFP.

Bangladeshi unions and rights activists also reacted furiously, calling for an end to the impunity accorded to manufacturers.

Tens of thousands of garment workers on Thursday held a protest at a nearby industrial area, forcing hundreds of factories to close for the day.

Low-cost British clothing chain Primark said one of its suppliers was based in the Rana Plaza, in the town of Savar, that collapsed at about 9am (0300 GMT) on Wednesday.

"The company is shocked and deeply saddened by this appalling incident at Savar, near Dhaka, and expresses its condolences to all of those involved," it said in a statement.

Walmart said it was investigating to see if any factories in the Rana Plaza building had been among its suppliers. Spanish fashion label Mango and Benetton of Italy said none of their Bangladeshi suppliers were involved.

Survivors say the building developed cracks on Tuesday evening, triggering an evacuation of the roughly 3,000 garment workers employed there, but that they had been ordered back to the production lines.

"The managers forced us to rejoin and just one hour after we entered the factory the building collapsed with a huge noise," said a 24-year-old worker who gave her first name as Mousumi.

Local police chief M Asaduzzaman told AFP that cases have been filed against the owner of the building, a ruling party official, and the garment factory owners for death due to gross negligence.

Mustafizur Rahman, head of a police unit created to handle industrial problems, said the factory owners were in hiding after ignoring a warning not to reopen the building.

"After looking at the cracks on Tuesday, we told them to keep the plants shut. They defied our call," he told AFP.