The Fourth Plenum brings together the party's 205-strong Central Committee and around 170 reserve members, along with officials from bodies including its much-feared internal watchdog, the Central Commission for Discipline Inspection.
The gathering, typically held at a Beijing hotel, is expected to take action against Zhou Yongkang, the powerful former domestic security tsar who fell to the anti-corruption campaign Xi launched with much fanfare after coming to power two years ago.
The Politburo announced in July that the plenum will focus on "governing the country according to the law" to ensure "economic growth, clean government, social justice", the official Xinhua news agency reported.
But the concept is seen very differently in Beijing than in the West, analysts say.
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In a commentary this week the state-run Global Times newspaper assured readers that rule of law will not challenge the "people's democratic dictatorship".
"For the party, 'rule of law' means what we would call 'rule by law': using the legal institutions -- procurators, courts, lawyers -- to continue to enforce one-party rule," said Andrew Nathan, a China scholar at New York's Columbia University.
The campaign against dissent has seen dozens detained, many for the nebulous crime of "picking quarrels and provoking troubles", and targeted even moderate critics of the Communist Party, including rights lawyers Xu Zhiyong and Pu Zhiqiang, to activists' alarm.