Chinese authorities have arrested the once-feared ex-security chief Zhou Yongkang and launched a criminal investigation on charges ranging from adultery and bribery to leaking state secrets, after expelling him from the Communist Party overnight.

The developments, announced Saturday shortly after midnight, pave the way for a trial of the most senior figure so far to be ensnared in President Xi Jinping's anti-corruption crackdown and appear to seal the downfall of a formerly powerful politician once considered a potent rival for Xi.

The square-jawed, granite-faced Zhou, 72, is the highest-level official to be prosecuted since the 1981 treason trial of Mao Zedong's wife and other members of the "Gang of Four" who persecuted political opponents during the 1966-76 Cultural Revolution.

He had been under investigation for "severe disciplinary violations" — a phrase is usually used to describe corruption — since July and presumably had been detained by party investigators months earlier. He had not been seen publicly since October 2013.

"He abused his power to help relatives, mistresses and friends make huge profits from operating businesses, resulting in serious losses of state-owned assets," the official Xinhua News Agency said.

Arrest follows expulsion from party

The party's central commission for discipline inspection said in a statement posted online that the decision to expel Zhou was made Friday at a meeting of the political bureau of the party's Central Committee. The attendees deliberated an investigation report on Zhou, who was in charge of China's massive domestic security apparatus before his retirement in 2012, and referred the case to prosecutors.

Shortly after the expulsion was announced early Saturday, prosecutors announced Zhou's formal arrest and opened a criminal case against him, the party said.

The investigation had found that Zhou had "seriously violated the Party's political, organizational and confidentiality discipline," Xinhua said.

"Zhou leaked the Party's and country's secrets," the Xinhua report went on to say, without revealing what he might have leaked, or to whom. "He seriously violated self-disciplinary regulations and accepted a large amount of money and properties personally and through his family. Zhou committed adultery with a number of women and traded his power for sex and money."

'Zhou leaked the Party's and country's secrets.' - Xinhua, China's official news agency

Any trial would be expected to have a foregone conclusion with Zhou convicted of his charges, because the outcomes of such high-profile trials are widely believed to be negotiated among top leaders ahead of time.

Zhou was considered by many observers and political experts to have been a key rival to Xi. By targeting Zhou, Xi showed the considerable power he has amassed since he took the helm of the party in November 2012.

Former members of the powerful Politburo Standing Committee had long been considered off-limits for prosecution in an unwritten rule aimed at preserving party unity. But Xi vowed to go after both low- and high-level officials in his campaign to purge the party of corruption and other wrongdoing that have undermined its legitimacy in the public eye.

Zhou was once perceived as untouchable, with expansive patronage networks covering the sprawling southwestern province of Sichuan where he was once party boss and controlled the state oil sector, police and courts. More significantly, as China's security chief, he oversaw the country's domestic spy agencies, a position that afforded him access to information on other high-ranking politicians who might pose a threat to him.