The Shanghai Women's Federation has found that more than 4,700 female workers have been subjected to working illegal overtime at 35 enterprises, according to the results of an inspection that the group released Friday.

The group looked into 445 companies from March 4 to March 11 during an investigation into the rights of female workers in the city. The group targeted companies in the clothing, textile, retail and catering industries, among others. About 139,000 employees work at those companies, 42.4 percent of whom are women.

Female workers at 17 clothing and textile factories often put in more than 60 hours per week and sometimes worked without a day off for months when the factories had urgent orders to complete, the women's federation said.

The group pointed out that the practice broke Chinese labor law and violated the workers' rights.

"The situation is prevalent nowadays because these companies seek out young female employees who are willing to work and save more money. They usually have little education and know very little about their rights," said Zhou Zhiquan, a labor lawyer and partner at the Shanghai office of the Beijing Yingke Law Firm.

In addition, the labor unions at these companies are usually weak, which makes it more difficult for individual workers to protect their rights. "Many workers keep silent because they are afraid they will get fired, which makes the situation even worse," Zhou told the Global Times.

Zhou advised overworked workers to keep records of their hours so they have evidence if they ever have to sue their employer.

"It is easier for blue-collar workers to collect evidence that they are being overworked than it is for white-collar workers, who often have to receive e-mails or do other jobs when they're supposed to be off," Zhou said.

Delayed pay was another problem the group found. About 6 percent of the companies that the women's federation inspected did not pay their 565 female workers on time. Most were small- and medium-sized private companies that paid their workers for several months of work at a time.

Zhou said it would take some time to fix these problems because they required major efforts from the government, the labor unions and the companies to ensure fairer treatment of workers.

In addition, some companies failed to provide female employees with a legally required medical exam every two years. According to the women's federation, the companies usually said they didn't know about the requirement.