The official Xinhua news agency reported that Zhou Yongkang's position as head of the Political and Legal Affairs Committee, a sprawling body that oversees law-and-order policy, had been taken over by Meng Jianzhu, the public security minister.
The hulking, grim-faced Mr Zhou, 69, had to retire along with most members of the Politburo Standing Committee, the inner council at the apex of power, at this month's 18th Party Congress, because of his age. He turns 70 in December.
Mr Meng, however, is only a member of the new Politburo, the 25-member body which reports to the downsized Standing Committee, putting him on a tighter leash and returning to a pattern the party kept to for much of the 1980s.
Reducing the party's Standing Committee from nine to seven members came as part of a once-in-a-decade leadership change announced last week, which saw the vice president, Xi Jinping, raised to head of the ruling Communist Party.
Mr Zhou had been on the Standing Committee since 2007 while also heading the central Political and Legal Affairs Committee.
That double status allowed Mr Zhou to dominate a domestic security budget of US$110 billion (Dh403.7bn) a year, exceeding the defence budget.
Mr Zhou was implicated in rumours that he hesitated in moving against the politician Bo Xilai, a former candidate for top office who fell in a divisive scandal after his wife was accused of murdering a British businessman.
Security forces also suffered a humiliating failure earlier in the year when they allowed blind rights advocate Chen Guangcheng to escape from 19 months of house arrest and flee to the US embassy in Beijing.