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.