BEIJING — Chinese authorities placed the feared ex-security chief Zhou Yongkang under formal arrest early Saturday to investigate his suspected crimes, including accepting bribes, adultery and leaking the country’s secrets, the official Xinhua News Agency said.

Zhou, a former member of the powerful nine-member Politburo Standing Committee, also was expelled from the Communist Party, making him the most senior figure to be snared in President Xi Jinping’s anti-corruption crackdown.

Zhou 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.”