The three-storey building used to manufacture veterinary medicines came crashing down, probably the result of a boiler and a gas cylinder explosion at the premises in the congested Multan Road area, police said.
Rescue workers dug through the rubble with bare hands, desperate to answer cries for help from people trapped beneath concrete slabs as sobbing relatives urged rescuers to do everything possible to recover their loved ones.
Police said the Orient Labs (Private) Limited factory was illegal as it did not have the proper registration, and they were hunting down the three partners in the business.
The accident at the 25-year-old factory will likely highlight poor safety procedures among Pakistani manufacturers and, with young boys among the dead, the use of child labour.
Senior police official Shoaib Khurram said that nine people were confirmed dead and 16 others wounded, as rescue workers turned on floodlights to carry on the recovery operation after dusk.
Officials identified the dead as seven women and two boys, aged 12 and 14.
An AFP correspondent saw rescue workers carry out one of the dead children and put his body into an ambulance to be driven to hospital.
Different officials estimated that there were between 35 and 50 people still buried under the rubble. Most of them were believed to be women and children, who were hired to pack up the medicines.