Bangladeshi relatives hold photos of the missing and dead workers three days after an eight-storey building collapsed in Savar
The death toll has risen to 337.
Around 40 people were pulled alive overnight from the ruins of the eight-storey Rana Plaza compound which caved in on Wednesday morning while thousands of garment workers were stitching clothes for Western brands.
But emergency workers warned their task was getting harder as any survivors still trapped in the rubble were now too weak to call for help.
News of the arrests of two factory owners as well as two engineers came after Prime Minister Sheikh Hasina vowed that those responsible for the country's deadliest industrial disaster would be hunted down and punished.
Bazlus Samad, the chairperson of New Wave Buttons and New Wave Style factories, and Mahmudur Rahaman Tapash, a managing director, were detained shortly after midnight, Dhaka's deputy police chief Shyaml Mukherjee told AFP.
Police had filed a case against them for "death due to negligence", said Mukherjee. The overall owner of the building had still to be traced, he added.
The senior investigating officer Kaiser Matubbor told AFP that two municipal engineers who had given the building the all-clear after an inspection on Tuesday night had also been arrested and would face similar charges.
New accusations
Survivors have said the building developed visible cracks on Tuesday, but bosses ordered staff to return to the production lines when they protested.
Five factories were based in the complex at Savar on the outskirts of Dhaka.
Bangladesh is the world's number two manufacturer of garments and the industry is a key driver of the economy. But it has a shocking safety record, with a fire at another factory killing 111 people in November.
British low-cost fashion line Primark and Spain's Mango have acknowledged having their products made in the block, while other brands including Walmart are investigating.
The accident has prompted new accusations from labour activists that Western companies place profit before safety by sourcing their products from Bangladesh where textile workers typically earn less than $40 a month.
Hundreds of relatives of missing workers have massed at the disaster site to watch the rescue teams try to find their loved ones.
National fire service chief Ahmed Ali said rescuers were having to battle exhaustion and a growing stench from decaying bodies.
Too weak
"We believe some people are still alive. But they are too weak to call for help," he told AFP.
Ali said rescue chiefs would meet with experts to decide whether to employ heavy lifting equipment to reach those still trapped in "some air pockets".
Teams of soldiers and firefighters have so far only used hand tools like cutters and drills, fearing heavier equipment could dislodge masonry.
An army spokesperson put the death toll at 337, with 2 417 pulled out alive since Wednesday.
Although the discovery of more survivors overnight gave fresh impetus to the rescue effort, there was growing anger at its pace.
"I've been here since Wednesday. We still don't know what happened to my aunt and sister-in-law," said Harunur Rashid.
"The rescue work is going on very slowly. There are too many people, yet too little work. Had they stepped up cutting of the concrete, I think they could save quite a lot of people," he said, clutching photos of his relatives.
Fugitive owner
Merina Begum, who was among those to be rescued early Saturday, said she and seven fellow workers trapped in the same area had been without any food or water since Thursday.
"When the building collapsed I just ran. Then I discovered myself on a dark floor with the roof just a few inches over my head. Seven other workers were also there," the 21-year-old told AFP before being whisked away for treatment.
"I managed to crawl and get a small bottle of water which all eight of us shared on the first day.
"This morning when the rescue workers brought juice, ice cream and cold water, it was tastiest food I've ever had."
With many of the country's 4 500 factories already shut due to protests, manufacturers declared Saturday a holiday and unions called a strike for Sunday.
Several thousand garment workers protested Saturday near the disaster site, demanding arrest of the fugitive owner of the building and there was also a solidarity protest by around 2 000 workers in the second city Chittagong. – Sapa-AFP