BEIJING — China’s leader, Xi Jinping, is pushing to establish a new anticorruption agency with sweeping powers to sidestep the courts and lock up anyone on the government payroll for months without access to a lawyer — a plan that has met surprisingly vocal opposition from some of the nation’s foremost legal minds.

The proposal is audacious even by the standards of the Chinese Communist Party, which is notorious for relying on secretive detention but has also proclaimed the rule of law as essential to a modern economy. Dozens of lawyers and law professors from China’s academic mainstream have risked retaliation by speaking out against the plan, in the first substantial public challenge to Mr. Xi’s second-term agenda.

During his first term, Mr. Xi waged a far-reaching campaign against graft, using it to imprison rivals, instill fear within the party establishment and set himself up as the nation’s most powerful leader in decades. Under draft legislation issued this month, a new National Supervision Commission would extend the reach of Mr. Xi’s campaign to millions more people, including those employed at universities and state-owned firms.

The nation’s current anticorruption watchdog is an arm of the Communist Party, with broad powers but jurisdiction only over the party’s 89 million members. Mr. Xi’s new commission would be a state agency with oversight over China’s entire public sector, which employs as many as 62 million people, many of whom do not belong to the party.

Opponents say the legislation would violate China’s Constitution and give the new commission carte blanche to operate beyond the scope of Chinese laws, especially those meant to prevent arbitrary arrest.

“There are parts of the draft supervision law that mark a clear retreat in protecting human rights,” Tong Zhiwei, a law professor from Shanghai, told a meeting of 40 or so like-minded law scholars in Beijing recently, according to a transcript that he shared. “The powers of the supervision commission would be too broad, and it lacks official checks on its power.”

By opposing the new commission, Professor Tong and others have raised broader questions about the strength and independence of the Chinese legal system, and whether it can be used to rein in the power of party leaders who have often set themselves above the law.

A party congress last month appeared to strengthen Mr. Xi’s authority and usher in a new era of strongman rule in China, laying out a vision of society under the all-encompassing control of the party. The opponents of the new commission are appealing to a counter-ideal: that everybody, including Mr. Xi, should be bound by the law.

“The party says it acts within the Constitution and law, but now the party also says it leads everything,” said Hong Zhenkuai, a historian in Beijing who has followed the debate. “How can you abide by the law if you also lead everything and are above the law? That’s the core problem with the supervision commission.”

Xi Jinping’s speech after the election of top leadership
VIDEO BY CGTN

Speaking out against the new commission has required courage in China’s harsh political climate, where criticism of major party initiatives is rarely tolerated. During Mr. Xi’s first five years as China’s leader, outspoken human rights attorneys were imprisoned and hauled out to make televised confessions, while Mr. Xi has denounced liberal ideas like constitutional government.

Many critics of the proposed commission are law professors who teach at prestigious schools in Beijing and Shanghai, and who have kept away from the front line of human rights cases. As they have come forward in recent weeks to criticize the new commission, they have striven to cast themselves in the role of a loyal opposition.

The lawyers and legal experts are treading a fine line by not challenging the rule of the Communist Party, but rather calling on the party to honor its own commitments to the rule of law.

Chinese leaders going back to Jiang Zemin in the 1990s have vowed to uphold the rule of law as part of making China a modern, developed nation. Even Mr. Xi has paid lip service to respecting the law and the Constitution, though he has also declared that “the party is the leader of all.”

The commission’s critics have voiced their opposition not in street protests, but via meetings of legal professionals, and in joint statements and legal commentaries online. But their criticisms are pointed nonetheless.

They say the new watchdog would violate the Constitution by creating, out of legal thin air, a new body whose vague powers would equal or even surpass those of China’s courts and legislature.

“This is a serious blow to the spirit of rule of law,” said Cheng Hai, a lawyer in Beijing who signed a petition criticizing the planned commission. “In principle, the human rights of every crime suspect should be protected.”

Some critics of the law have played a cat-and-mouse game with censors, posting harsh criticisms of the commission on WeChat, a popular Chinese social media service, that are quickly taken down.

“If there’s no freedom to criticize, then soliciting opinions is completely meaningless,” said one comment, which was also removed by censors.

Partly, the legal professionals’ opposition reflects a feeling of betrayal. When the Chinese government first proposed the new commission last year, some lawyers hoped that it would put Mr. Xi’s anti-graft drive on a firmer, fairer legal foundation.

“Initially, we were all excited about the commission,” said Fu Hualing, a law professor at the University of Hong Kong.

China already has a party-run anticorruption agency. Called the Central Commission for Discipline Inspection, it is a secretive operation that hands suspects over to prosecutors only after detentions without appeal or access to lawyers.

After Mr. Xi became party leader in 2012, he placed his ally Wang Qishan in charge of the party’s anticorruption agency, which moved aggressively to take down dozens of party officials at the highest ranks. The anticorruption drive helped cement Mr. Xi’s firm control, but it also spawned accusations of abuse and torture.

Last week, the agency announced that Lu Wei, the brash former head of China’s top internet regulator, was under investigation, declaring that his fall showed Mr. Xi’s anticorruption drive would not rest.

The new commission would take over from the party watchdog, but with broader investigative reach. While the current agency’s focus has been limited to Communist Party members, the new commission would be empowered to examine anyone on the public payroll, including the millions of government employees who are not party members.

The new commission “will strengthen the leadership of the party through the form of rule of law,” Fan Peng, a politics researcher at the state-run Chinese Academy of Social Sciences, wrote in a recent online commentary.

Wang Qishan steps down from China’s top leadership
VIDEO BY SOUTH CHINA MORNING POST

The new commission will also have the power to detain people for three months in secret, with the possibility of a three-month extension. This will give it the same draconian powers of detention that the current party watchdog uses solely against party members.

“This removes the fig leaf of a divide between the party and the state,” Carl Minzner, a Fordham University professor who studies Chinese law and politics, said of the draft law. “Instead of it being a step toward imposing greater legal constraints, I think it arguably represents the partial absorption of the legal system by the party apparatus.”

The National People’s Congress has invited comments until Dec. 6 on the proposed law to create the commission. But the congress is packed with officials handpicked for their loyalty to the party, and they are unlikely to delay or overhaul the plans. The legislation is likely to pass when the congress meets for its next annual session, probably in March next year.

Most opponents see little hope of delaying the new commission’s creation, but they said they still hope to blunt what they call the most dangerous shortcomings.

“That decision is up to the party central leadership,” Professor Tong, the Shanghai academic, said. “As legal experts, and as citizens, we’re just doing everything we can to fulfill our duty to offer our opinions.”