Image copyright Getty Images Image caption China's crackdown on corruption is about to get even tougher

Tackling China's rampant corruption has been a personal mission of Xi Jinping's since he became president, and his campaign is now moving up a gear with the establishment of a new anti-corruption agency overseeing millions of people.

The National Supervision Commission (NSC) will oversee "all public servants exercising public power" - not just party members. It is thought the agency will supervise about three times as many people as existing watchdogs.

But critics say the greatly increased powers held by this new body - which rank it higher than the supreme court - give real cause for concern.

Amnesty International has called it "a systemic threat to human rights in China".

What is the new anti-corruption agency?

China taking aim at corruption is nothing new. More than a million Communist Party officials have been disciplined in recent years.

Image copyright AFP Image caption Is President Xi fighting corruption or critics?

But it's the scope that's now changed. Until now, the main watchdog was the Communist Party's Central Commission for Discipline Inspection (CCDI).

It would sweep in to investigate allegations of bribery, embezzlement or other corruption, removing guilty people from their posts and sending them to court. There they would get jail terms or in extreme cases, the death penalty.

Among the biggest names to fall under Mr Xi's campaign have been Zhou Yongkang, once the third most senior leader in China, and Bo Xilai, the former party chief of Chongqing who once seemed destined for senior leadership.

Given that most officials are also party members, the CCDI already had a wide reach - yet there were still many low-ranking state employees that escaped its jurisdiction.

The newly formed NSC - also referred to as the State Supervisory Commission - will go beyond that and cast its watchful eye over all management-level public servants, including at places like hospitals and schools.

Image copyright AFP Image caption Mr Yang has spent years taking on corruption within the Communist Party

It will be led by Yang Xiaodu, who for the past few years has been the deputy chief of the CCDI.

"The NSC is very much likely to work in tandem with the CCDI," Tom Rafferty of the Economist Intelligence Unit told the BBC.

"Its establishment marks the beginning of an effort to enforce much more widely the drive against corruption, extravagance and indiscipline that we have seen within the party over the past five years."

The agency ranks higher than the supreme court and will be in charge of supervision, investigation and also punishment.

It will have the authority to interrogate and detain any government manager, as well as freeze his or her assets and seize their property.

Image copyright AFP Image caption This official dubbed 'Brother Watch' for his luxury wristwatch collection was jailed for corruption shortly after Xi Jinping came to power

With its unprecedented power comes also a change in the detention system.

Previously, a shadowy and extra-legal system called shuanggui was in place - an internal disciplinary procedure where party members were imprisoned for months without access to lawyers or family until they confessed.

Under the NSC, the name has been changed to liuzhi and is now an official legal procedure. Liuzhi still allows for suspects to be held for up to six months without legal counsel.

The Communist Party says the NSC will have the power needed to fight corruption, and that it will be subject to strict internal and external oversight.

Why is it controversial?

The existing campaign been described by some as a massive internal purge of opponents, on a scale not seen since the days of Mao Zedong and the Cultural Revolution.

Rights groups say the NSC's broad jurisdiction - along with the fact that Xi Jinping is now cleared to be president for life - is cause for concern.

A group of Chinese lawyers even sent a rare open letter last year warning that it would be a crisis for the rule of law in China, and urging proper legal oversight.

Critics worry defendants will be left with little or no ability to appeal against the agency's decisions and that without proper oversight, it could be open to manipulation for political ends.

Nicholas Bequelin, East Asia director at Amnesty International in Hong Kong, said the changes are "systemic threat to human rights".

"It places tens of millions of people at the mercy of a secretive and virtually unaccountable system that is above the law," he said in a statement.

"It bypasses judicial institutions by establishing a parallel system solely run by the Chinese Communist Party with no outside checks and balances. The law eviscerates China's legal system."

BBC Monitoring's Pratik Jakhar contributed to this report