Ultimately, though, the Communist Party brushed those objections aside. The new body was given formal rubber-stamp approval by the National People's Congress at the closing session of its annual meeting today.
The parliament already has voted to alter the constitution to accommodate the National Supervision Commission (NSC) as part of changes that also abolished term limits on the presidency.
On Sunday, the parliament installed Yang Xiaodu, who is believed to be a protege of the president, as the first head of the NSC.
Xi's campaign to tackle endemic corruption within the Communist Party is thought to have won him popular support. But it has also served an important dual purpose: to impose his will on the Communist Party and to root out rivals.
The Central Commission for Discipline and Inspection, the agency tasked with battling corruption within the party, will form part of the new NSC.
It says it has disciplined more than 1.5 million officials since a campaign against graft began in 2012, taking down many high-profile figures such as Zhou Yongkang, the once-feared former head of the security services now serving life in prison; Ling Jihua, chief of staff to former president Hu Jintao; and some of the army's most senior generals.
"China's wisdom to crack down on corruption, a hard nut that troubles the entire globe, serves as an important inspiration to the world," the state-run People's Daily newspaper said yesterday, arguing that other developing countries have found that corruption worsened after they adopted Western-style democracy.
"Given such a backdrop, China's bold reform has shed a new light for the world."
Yet for many lawyers, a community that has suffered an unprecedented crackdown under Xi's rule, the light of due process, and of a transparent and fair legal system, is being extinguished.
The anti-corruption campaign often extracted confessions through "shuanggui," a system of detention outside the legal process in which suspects were held incommunicado, and sometimes tortured, in secret locations for months.
The new law replaces shuanggui with a new system called "liuzhi," which is supposed to be subject to stricter controls, promising detainees adequate food and rest, for example. But in practice, it is unlikely to be any different, lawyers and human rights groups say.
Detainees can be held without access to lawyers for up to six months, while the right to inform families of a detainee's whereabouts can be waived if investigators think that doing so would harm their investigation.
"Public power must operate in an open, fair and equitable manner to prevent abuse," 59 lawyers and jurists wrote in an open letter on the proposals last year, calling the law a "watershed moment between sticking to the rule of law and moving toward the rule by man."
The group received no response to the letter, said Cheng Hai, a Beijing lawyer who helped draft it. Other lawyers who voiced objections had their comments scrubbed from the Internet or were later persuaded to publicly back the proposals.
Tong Zhiwei, a lawyer who had earlier written about his concerns, was not given permission by East China University of Political Science and Law to talk to the Washington Post. But in previous essays, he argued that the proposals would weaken the power of the judiciary to defend social justice, would infringe on the basic idea embodied in China's constitution of "equality before the law" and would not be effective.
"Anti-corruption is a project that belongs to the whole population," he wrote, adding that the new agency effectively barred society from taking part in the exercise. "Anti-corruption without the participation of the masses cannot be effective."
Research by political scientists Dimitar Gueorguiev and Jonathan Stromseth showed that the more provinces within China had empowered citizens to monitor the bureaucracy - by increasing public access to government budgets and decision-making - the more success they had in fighting graft.
By contrast, the new agency is explicitly based on a top-down "political" approach, they wrote in an article for the Brookings Institution. That leaves it "vulnerable to political currents and manipulation" and could dampen public participation in the fight against corruption.
But it does help to fulfill one of Xi's key goals, experts say: reimposing Communist Party control over every aspect of public life in China, partly through fear.
"This is a lever to instill the entire Chinese bureaucracy with a heightened sensitivity to the wishes of the centre - and in the new area that is dawning, that is synonymous with Xi Jinping," said Carl Minzner, a professor at Fordham Law School.
Party discipline regulations specifically outlaw any "improper discussions of central party policy." Expanding that into universities and think-tanks, Minzner warned, "risks further curtailing what limited room still exists for objective discussion and analysis of state policies."
In the short run, this approach could make it easier for Xi to impose his will.
For example, China's success in curbing air pollution last year came about partly because the Ministry of Environmental Protection had the backing of the anti-corruption authority, pointed out Andrew Polk of the Trivium consultancy in Beijing.
As a result, the ministry's inspectors, previously often ignored, were now feared, he said.
But too much fear can make officials scared to take risks, or to make any decision, unless explicitly ordered to do so - and then overzealous in carrying out those orders, experts warn.
Nicholas Bequelin, East Asia director at Amnesty International in Hong Kong, said the new commission marked "the end of rule of law" in China, eviscerating legal institutions by establishing a parallel system run directly by the Communist Party, with no checks and balances outside the party itself.
"China is putting in place a terrifying machinery that is designed to produce forced confessions out of anyone the party chooses to investigate, rightfully or not, for a wide range of actions that range from corruption to one's 'political stance,' " Bequelin said.
"The proven tools to address corruption are transparency, accountability and an independent judiciary. The supervision commissions set by this law are secretive, unaccountable and above the law."
