BEIJING - The criminal case against China's ex-security chief not only plays to public demands to curb corruption but spells the downfall of one of President Xi Jinping's biggest rivals, puts other challengers on their toes, and leaves Xi more solidly in control than ever.

The fate of the once-feared Zhou Yongkang, 72, appeared to be sealed by the just-after-midnight announcements Saturday that he was expelled from China's ruling Communist Party and arrested in a criminal investigation into allegations ranging from bribe-taking to leaking state secrets.

"Many of Xi's enemies have been scared, and he's been successful in intimidating his enemies," said Willy Lam, an observer of China's elite politics at the Chinese University of Hong Kong. "All of them have become obedient - at least superficially - to Xi Jinping."

Zhou, with a face that looks like it is made of stone, was a former member of the party's all-powerful Politburo Standing Committee and was once in charge of the country's police, security forces and judiciary, a vast apparatus that spends more on domestic security than what China spent on the People's Liberation Army, the world's biggest military.

Zhou's status as security czar would have meant he had access to private phone conversations and secret information about national leaders. The state secrets allegations against him likely stem from his attempts to use leaks about colleagues to jockey for position ahead of China's handover of power in late 2012 to a new generation of leaders at the retirement of President Hu Jintao, Lam said.

If he ends up formally charged on that count, it also may give the court a reason to keep trial proceedings closed and thus limit any politically damaging public disclosures.