Electronics giant Samsung has been accused of illegally using child labour and mistreating workers in China.
The US-based China Labour Watch said its investigation into conditions at eight factories in China showed some employees were working more than 100 hours a month overtime and that children were knowingly employed.
The watchdog also said their investigation of the six Samsung-owned plants and two suppliers showed that safety measures, such as providing workers with protective clothing, were not followed.
The group also accused Samsung of barring workers from sitting during their shifts and said it had documented instances of physical and verbal abuse.
The South Korean company responded swiftly to the allegations, acknowledging that poor working conditions may have arisen due to production demands and that a review was to immediately take place.
"We frequently review our manufacturing facilities regarding overtime work. We will re-evaluate working hour practices," said a spokesman. "When new production lines are completed or new products are launched, high demand has led to overtime work."
But the company denied allegations that it used child labour, saying it has "zero tolerance" for the hiring of underage workers.
According to China Labour Watch, overtime for some employees reached or exceeded 100 hours per month, with some employees having only one day off a month.
The report also said that while Samsung suppliers Tianjin Intops and Tianjin Chaarmtech Electronics complied with minimum wage laws - workers were paid the base monthly salary of 1,310 renminbi - the pay was so low that many were compelled to work overtime.
"Dependence on overtime work is characteristic of workers at almost every investigated factory," the report said.
The group also claimed that Samsung or its suppliers knew that Chinese underage youths were using forged identity cards to get jobs. Hiring children under the age of 16 is illegal in China.
The group's report said that "the factory did not take any actions to prevent the hiring of child workers even when they know about them." Samsung also failed to provide a way for workers to lodge complaints.
"Even when they suffered unfair treatment, workers at almost factory lacked any effective channel by which to express grievances to management," the report said.
China Labour Watch said it investigated eight plants in China that produce phones, media players, DVD players, TV components, mobile displays, printers, home appliances and phone casings for Samsung. They hire more than 24,000 workers combined.
Investigators entered the factories undercover or interviewed workers outside the factories.
The allegations faced by Samsung are reminiscent of a labour scandal that recently dogged Apple.
A spate of worker suicides at Foxconn Technology Group, Apple's largest supplier, drew attention to conditions faced by workers in China who put iPhones and other devices together.
The world's two largest smartphone makers are embroiled in patent fights in 10 countries. Last month, a jury hit Samsung with a one billion dollar judgment after finding the company had wilfully copied Apple's iPhone and iPad. Samsung is appealing.