Passage of the measure came shortly after officials and state media unearthed the widespread use of slave labor in as many as 8,000 brick kilns and small coal mines in Shanxi and Henan provinces, one of the most glaring labor scandals since China began adopting market-style economic policies a quarter century ago.

Police have freed nearly 600 workers, many of them children, held against their will in factories owned or operated by well-connected businessmen and local officials.

Moreover, the law empowers company-based branches of the state-run union or employee representative committees to bargain with employers over salaries, bonuses, training and other work-related benefits and duties.

In the past, workers have had to negotiate wages with their employers individually. China’s state-run union has had almost no involvement in setting wage and benefit levels.

The Communist Party’s monopoly union, known as the All-China Federation of Trade Unions, is a legacy of China’s socialist planned economy. It is an official state organization charged with overseeing workers that in practice has tended either to play no role whatsoever or to help managers monitor and control workers.

Workers are not allowed to form their own, independent unions. The state union rarely if ever presses for higher wages or enhanced benefits. It does not permit strikes....

International labor experts said that several of the most delicate clauses had been watered down. But lawyers representing some big global companies doing business here complained today that the new law still imposes a heavy burden.

“It will be more difficult to run a company here,” said Andreas Lauffs, a lawyer at Baker & McKenzie, which represents many of America’s biggest corporations in China.