China’s leader, Xi Jinping, is pushing to establish a new anti-corruption 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 Xi’s second-term agenda.

During his first term, Xi waged a far-reaching campaign against graft, using it to imprison rivals, instil 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 Xi’s campaign to millions more people, including those employed at universities and state-owned firms.

The nation’s current anti-corruption watchdog is an arm of the Communist Party, with broad powers but jurisdiction only over the party’s 89 million members. 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, 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 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 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.”

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 Xi’s first five years as China’s leader, outspoken human rights attorneys were imprisoned and hauled out to make televised confessions, while 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 criticise 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 honour 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 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.

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,” 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.”