More than two days after their factory collapsed on them, at least some garment workers were still alive in the corpse-littered debris today, pinned beneath tons of mangled metal and concrete.

As the death toll passed 300, rescue crews struggled to save them, knowing they probably had just a few hours left to live, as desperate relatives clashed with police in their anger and grief.

Amid the chaos, the cries for help and the smell of decaying bodies at the eight-storey building, what happened to 18-year-old Mussamat Anna passes as luck. Rescue workers cut off the garment worker's mangled right hand to pull her free from the debris.

"First a machine fell over my hand and I was crushed under the debris. ... Then the roof collapsed over me," she said from a hospital bed.

Military spokesman Shahin Islam told reporters that 304 bodies had been recovered.

Brig Gen Mohammed Siddiqul Alam Shikder, who is overseeing rescue operations, said 2,200 people have been rescued. The garment manufacturers' group said the factories in the building employed 3,122 workers, but it was not clear how many were inside it when it collapsed on Wednesday in Savar, a suburb of Bangladesh's capital, Dhaka.

An army rescue worker, Maj Abdul Latif, said today that he found one survivor still trapped under concrete slabs, surrounded by several bodies. At another place in the building, four survivors were found pinned under the debris, a fire official said.

An AP cameraman who accompanied a rescue crew heard two men's anguished cries for help; it was unknown whether they were still alive today.

Rescue workers said they were proceeding very cautiously inside the crumbling building, using their hands, hammers and shovels, to avoid more injuries and collapses. But they said the trapped workers were so badly hurt and weakened that they would need to be extricated within a few hours if they are to survive.

Maj. Gen. Chowdhury Hasan Suhrawardy told reporters that search and rescue operations would continue until at least tomorrow.

"We know a human being can survive for up to 72 hours in this situation. So our efforts will continue nonstop," he said.

Police cordoned off the building site, pushing back thousands of bystanders and relatives, after rescue workers said the crowds were hampering their work.

Clashes later erupted between relatives of those still trapped and police officers, who used batons to disperse the mobs. Police said 50 people were injured in the clashes.

"We want to go inside the building and find our people now. They will die if we don't find them soon," said Shahinur Rahman, whose mother was missing.

Elsewhere, many thousands of workers from the hundreds of garment factories across the Savar industrial zone and other nearby industrial areas took to the streets to protest at the collapse and poor safety standards.

Local news reports said protesters smashed dozens of vehicles at one strike.

Police say cracks in the building had led them to order an evacuation on Tuesday, but the factories ignored the order and were operating when it collapsed.

Video shot before the collapse shows cracks in the walls, with apparent attempts at repair. It also shows columns missing chunks of concrete and police talking to building operators.

Officials said soon after the collapse that numerous construction regulations had been violated.

Abdul Halim, an official with Savar's engineering department, said the owner of Rana Plaza was originally allowed to construct a five-storey building but added another three stories illegally.

Mahbubul Haque Shakil, a spokesman for Prime Minister Sheikh Hasina, said she had ordered police to arrest the owner of the building as well as the owners of the garment factories in "the shortest possible time."

Local police chief Mohammed Asaduzzaman said police and the government's Capital Development Authority have filed separate cases of negligence against the building owner.

Bangladesh's garment industry was the third-largest in the world in 2011, after China and Italy. It has grown rapidly in the past decade, a boom fuelled by Bangladesh's exceptionally low labour costs. The country's minimum wage is now the equivalent of about 38 dollars a month.