The Chinese Communist Party's decision to arrest and expel former security chief Zhou Yongkang is a bold step that demonstrates President Xi Jinping's determination to consolidate power "to a degree unseen" in decades, observers said Saturday.
Zhou -- who retired from China's all-powerful Politburo Standing Committee (PSC) in 2012 -- has been placed under a judicial probe for a barrage of charges including bribetaking and "leaking state secrets", the official Xinhua news agency reported.
The announcement early Saturday makes Zhou the most senior Communist Party official to be investigated since the infamous Gang of Four -- a faction that included the widow of founding leader Mao Zedong -- were put on trial in 1980.
In pursuing charges against Zhou, Xi "is breaking long-standing internal norms that had exempted the very top level of Chinese Party leaders from prosecution after they had left office", said Carl Minzner, an expert on Chinese law at Fordham Law School.
Zhou was a close ally of disgraced politician Bo Xilai, whose hard-charging approach led to his ouster from the party's top ranks -- a factor that experts say contributed to Zhou's downfall.
"In toppling... Zhou, Xi has solidified his power to a degree unseen since the beginning of the reform period" in the late 1970s, Minzner told AFP, adding that "the rules of the game have been changed, quite dramatically".
A key figure in China's powerful petroleum industry, Zhou became ensnared in Xi's much-publicised anti-corruption drive in July when he was put under investigation for "serious disciplinary violation".
Xinhua said the decision to expel him was made at a Politburo meeting on Friday, indicating that the move was approved by the party's innermost circle of leaders, including Xi.
- 'Power for sex and money' -
Communist Party authorities have been waging an anti-graft campaign since Xi ascended to the leadership two years ago.
Official graft has caused widespread public anger in China, and since taking office Xi -- who proclaimed it a threat to the ruling party's existence -- has sought to present himself as a crusader against the scourge.
The campaign has netted high-level "tigers" as well as low-level "flies" -- although critics say the Communist Party has failed to introduce systemic reforms to prevent graft, such as public disclosure of assets.
According to the Xinhua report, which cited a Politburo statement, Zhou "abused his power to help relatives, mistresses and friends make huge profits from operating businesses, resulting in serious losses of state-owned assets".
In unusually frank language, the news agency also said that Zhou was found to have "committed adultery with a number of women and traded his power for sex and money".
Adultery is not illegal in China, but the Communist Party has for years been embarrassed by reports of its cadres keeping multiple mistresses.
Authorities said in June that officials guilty of affairs "could be removed from their posts, or stripped of party membership".
On China's online social networks, which are closely monitored by Communist Party censors, news of Zhou's expulsion was widely shared, with the Xinhua report on his arrest receiving more than 30,000 "likes" by Saturday afternoon.
Discussion of the case was tightly restricted, however, with fewer than two dozen responses allowed to the Xinhua story and comments critical of the ruling party apparently scrubbed.
- A Bo Xilai-like trial? -
Zhou's expulsion from the party had been rumoured for months, and a number of officials with close ties to the former security czar have recently been ousted from the party.
They include Ji Wenlin, Yu Gang and Tan Hong, all former secretaries to Zhou, according to overseas Chinese reports.
The dismissal of an official from the party clears the way for a criminal prosecution which usually leads to a guilty verdict at a trial, followed by a prison sentence.
Jean-Pierre Cabestan, an expert on Chinese politics and law at Hong Kong Baptist University, said authorities would be wary of allowing a public trial for Zhou.
"There's always a risk with a public trial," he said, adding that party leaders may move to ensure that a Zhou trial is "more controlled" than the dramatic Bo proceedings, which included descriptions of a love triangle and details of the vast wealth amassed by the party's upper echelon.
Bo, who once headed the southwestern megacity of Chongqing, was sentenced last year to life in prison after a trial that exposed the lavish lifestyles of the party elite.
In addition to Zhou, Xi's anti-graft campaign has also led to the ousting of Xu Caihou, a former vice-chairman of China's Central Military Commission.
Xu, who was a Politburo member until 2012, in July became the first of the body's former members to fall in the current crackdown on graft, which follows the ascension of Xi to power as the head of the party.
Despite the crackdown, a recent report by Berlin-based Transparency International suggested that corruption has actually worsened in China, in part because "too many cases take place behind closed doors".
Michael Davis, a law professor at the University of Hong Kong, said that despite the anti-corruption drive, "party leaders really remain above the law".
"Generally, people in the world see that corruption is fought by free speech, a free press and transparency," he said. "But none of that seems to be in play here."
"We still don't see a real willingness to submit party leaders to the law in the normal fashion," he added.