SAVAR, Bangladesh (AFP) - Bangladesh made a string of arrests on Saturday over the collapse of a factory complex which killed at least 351 people as exhausted rescuers battled to find more survivors among the corpses.

Twenty-nine people were pulled alive on Saturday from the rubble of the eight-storey Rana Plaza compound which caved in Wednesday morning while thousands of garment workers were stitching clothes for Western brands.

Emergency workers, gulping breaths of air freshener to mask the stench of rotting bodies, warned their task was getting tougher as survivors were losing their strength to call for help.

"There are many dead bodies but our top priority is finding those who may still be alive," said Mahbubur Rahman, the fire service's director of operations.

"There are some survivors. We can hear their feeble cries or hear them talking to each other," he told AFP.

Police arrested three factory owners and two engineers after Prime Minister Sheikh Hasina vowed those responsible for the country's deadliest industrial disaster would be hunted down.

Proprietors Bazlus Samad and Mahmudur Rahman Tapash were detained soon after midnight while Aminul Islam, owner of two factories based in the doomed building, was arrested Saturday night Dhaka police spokesman Masudur Rahman told AFP.

They faced preliminary charges of causing death by negligence, police said Five factories were based in the complex at Savar, just outside Dhaka.

The government has launched a massive search for owners of two other factories and the overall proprietor of the complex, reportedly a ruling party official, after a minister alleged he built the compound without permits.

The local government deputy minister, Jahangir Kabir, identified the compound owner as Sohel Rana as police reported the man had gone into hiding.

Another building near the disaster site Rana allegedly owns was sealed after cracks appeared in the structure, Dhaka district administrator Yusuf Harun told AFP.

Senior investigating officer Kaiser Matubbor said two municipal engineers who gave the building the all-clear after an inspection on Tuesday were also arrested and could face charges of death due to negligence.

Survivors said the complex developed cracks Tuesday, but bosses ordered staff to return to the production lines.

Thousands of relatives of missing workers massed at the site to watch as bodies were pulled from the debris and laid on a school ground for identification.

Dhaka police officer Badrul Alam told AFP the "death toll is now 351", but was set to rise as more bodies could be seen.

There is no official figure of the number of people still missing.

Akram Hossain, a deputy director of the fire service, said there were still some survivors trapped between the pancaked floors but warned their chances were "diminishing by the minute" in the sweltering tropical heat.

Rahman, the fire service's director of operations, said rescuers were trying to carve tunnels through the wreckage and bodies could be seen everywhere.

"Pillars and beams are the biggest problem. Sometimes, even if we can locate survivors, we can't reach them because of these beams. They take a lot of time to cut through," he said.

Rescuers were only using hand tools like cutters and drills, fearing heavier equipment could dislodge masonry.

Although the discovery of more survivors gave fresh impetus to the rescue effort, there was mounting anger over its slow pace.

"I've been here since Wednesday. We still don't know what happened to my aunt and sister-in-law," said Harunur Rashid, clutching photos of his relatives.

"There are so many people, yet too little work," he said.

Merina Begum, among those rescued on Saturday, said she and seven fellow workers trapped in the same area had been without food or water for days.

"When the rescuers brought juice, ice cream and cold water, it was the tastiest food I've ever had," she told AFP.

With many of Bangladesh's 4,500 factories shut due to protests, bosses declared Saturday and Sunday a holiday.

Several thousand garment workers protested on Saturday near the disaster site but they were dispersed by police firing rubber bullets and tear gas.

Bangladesh is the world's number-two garment manufacturer and the industry is the mainstay of the economy. But it has a shocking safety record, with a factory fire killing 111 people in November.

Britain's Primark and Spain's Mango have acknowledged their products were made in the block, while other brands including Walmart are investigating.

The accident has prompted new accusations from activists that Western firms place profit before safety by sourcing their products from a country where textile workers often earn less than $40 a month.