An ally of China's President Xi Jinping has been appointed as Beijing's acting mayor as the ruling Communist party reshuffles officials before a key congress next year.

Cai Qi will replace Wang Anshun who has resigned as mayor of China's capital, the official Xinhua news agency said Monday, citing the city's rubber-stamp legislature.

The careers of Cai and Xi have overlapped, with both serving in the provinces of Fujian and Zhejiang. The South China Morning Post newspaper described Cai as a "close ally" of the president.

The party at its congress next year is expected to replace most members of its top decision-making body, the politburo standing committee.

Analysts say Xi and other top officials are jockeying to have allies promoted to key positions within the party and government in an opaque process of backroom bargaining.

The party last week declared Xi the "core" of its leadership, elevating his already powerful status as its general secretary.

Xi has sought to centralise power since assuming the party's helm in 2012, taking control of a series of key policymaking groups and drawing comparisons with Mao Zedong.