The government must strive to ensure that migrant workers have more equal access to public services and social welfare, and it must simultaneously reduce the development gap between urban and rural areas so that more of the rural population can find work opportunities at home, Tong said.

Zhang Jun, an independent labor rights activist in Yantai, Shandong province, said the most frequent infringements of migrant workers' rights are a failure to compensate for workplace injuries and unpaid wages.

Workers who claim to have been injured on the job have to follow a legal and medical process, Zhang said.

Under law, they must provide reams of documentation, including proof of employment and medical records, before they can apply for compensation.

"Many workers can't wait that long for the compensation so they just agree on a small amount of money from their employers and go back to their hometowns," he said.

Zhang said he dealt with a case where it took nearly two years for a worker in Yantai to get his compensation because he had not signed a labor contract with the employer.

Without a contract, it's hard to prove the employment relationship, and the employer also has rights to contest and delay lawsuits, he said.

Only 33 percent of construction workers have labor contracts with employers, and 78 percent of those do not have a copy of the contract, a recent survey by Beijing Normal University's China Labor Studies Center found.

Under law, employers must pay work-related injury insurance for employees, and if they do not, they are held responsible for injured employees' medical care and compensation.

"Wage delays are very common on construction sites, and some contractors even vanish after a project is finished, skipping out on paying the workers," Zhang said.

In the construction industry, investors hire a contractor, who usually parcels out work to subcontractors, who also may use any number of subcontractors.

Most construction workers find work through acquaintances, and they worry they might lose the job by asking the employer for a contract.

Ye Jingyi, a labor law professor at Peking University, said labor authorities must tighten law enforcement and ensure that workers sign contracts with employers.

"Workers need to get a better understanding of their legal rights in the workplace and what to do if they are denied their rights," she said.

"Labor authorities must punish employers who do not sign contracts with workers or violate other labor laws."