The three-storey building used to manufacture veterinary medicines came crashing down after a probable boiler and a gas cylinder explosion at the premises in the congested Multan Road area on Monday, police said. Rescue workers spent the night digging through the debris with their bare hands, increasingly desperate as trembling cries for help started to recede from mostly women and children trapped beneath concrete slabs. Workers and volunteers used everything they could -- hammers, axes, chisels and shovels -- to shift the rubble and pull out the injured, coated in dust. "We hope to clear most of the rubble by tonight," local rescue chief Rizwan Naseer told reporters, saying that workers were digging tunnels under the rubble to pull out more injured and dead bodies. "It is a very slow and difficult operation," Naseer said, adding it took almost five hours to pull out two women alive overnight. "We have to be very careful because it involves human lives." At least 13 injured people have been pulled out alive and 17 bodies found -- that of 11 women, three young girls and three boys aged 12 to 16 years. The death toll is thought likely to rise further with dozens of people still believed to be trapped under the concrete mass. Police said the factory was illegal. Local residents said it had been shut down twice since 2008, but that the owners reopened the premises each time. "The owners violated the court orders and broke the seals," said top local administration official Ahad Cheema. The accident at the Orient Labs (Private) Limited factory spotlighted poor safety procedures among Pakistani manufacturers and the use of child labour. There was brief jubilation overnight when rescue workers recovered a 10-year-old boy alive, cheered on by chants of "Allahu Akbar" (God is greater) under generator-powered flood lights. "We peeped through a hole drilled in one of the concrete slabs and saw a boy. He talked to us, we pulled him out," rescue worker Mohammad Asif told AFP. "He had no serious injuries but he fainted in shock after seeing the scene." Most of those trapped under the rubble were believed to be women and children hired to package the medicines. Mohammad Akram, 50, said he ran to the site shortly after the building collapsed, desperately worried about his 10-year-old son, Asad, a packer. "I came here rushing and weeping but luckily my son has been rescued. He is injured and alive. But my 12-year old nephew is still missing." Regular working hours ran from 8:00 am to 5:00 pm, but employees were often required to work late into the night as well, Akram said. A small group of about 15 to 20 workers protested at the site on Monday, carrying the red flags of Pakistan's Labour Party and shouting: "Who is accountable for the (deaths) of these innocent workers? We want an answer."